From: https://github.com/dekeonus/libjwt/tree/feat-multi-ssl

Enables multiple ssl implemtations to be installed at the same time
default libjwt will default to linking to openssl (if openssl enabled)


diff --git a/configure.ac b/configure.ac
index 2b28327..183f163 100644
--- a/configure.ac
+++ b/configure.ac
@@ -8,25 +8,99 @@ AC_CONFIG_HEADERS([libjwt/config.h])
 
 AC_SUBST([AM_CFLAGS], [-Wall])
 
-dnl Prefer OpenSSL unless asked to ignore it
+dnl Build with OpenSSL when asked
 AC_ARG_WITH([openssl],
-	AS_HELP_STRING([--without-openssl], [Ignore presence of OpenSSL libraries and use GnuTLS]))
-
-AS_IF([test "x$with_openssl" != "xno"], [
-	PKG_CHECK_MODULES([OPENSSL], [openssl >= 0.9.8])
-	AM_CONDITIONAL([HAVE_OPENSSL], [true])
-], [
-	PKG_CHECK_MODULES([GNUTLS], [gnutls >= 3.5.8])
-	AM_CONDITIONAL([HAVE_OPENSSL], [false])
+	AS_HELP_STRING([--with-openssl], [Build using OpenSSL library]),
+	[with_openssl=$withval],
+	[blank_openssl=yes])
+
+dnl Build with GnutLS when asked
+AC_ARG_WITH([gnutls],
+	AS_HELP_STRING([--with-gnutls], [Build using GnuTLS library]),
+	[with_gnutls=$withval],
+	[blank_gnutls=yes])
+
+dnl If no with[out]-'ssl' default to openssl
+AS_IF([test "x$blank_openssl" = "xyes" -a "x$blank_gnutls" = "xyes"], [
+	with_openssl=yes
+])
+
+dnl emulate previous build, without-openssl means build with gnutls
+AS_IF([test "x$with_openssl" = "xno" -a "x$blank_gnutls" = "xyes"], [
+	with_gnutls=yes
+])
+
+AC_ARG_ENABLE([multi-ssl], AS_HELP_STRING([--enable-multi-ssl],
+		[Enable multiple ssl library implementations]),
+	[enable_multi_ssl=$enableval],
+	[enable_multi_ssl=no])
+
+AS_IF([test "x$enable_multi_ssl" = "xno"], [
+	AS_IF([test "x$with_openssl" = "xyes" -a "x$with_gnutls" = "xyes"], [
+AC_MSG_ERROR([You need to enable multi-ssl to support multiple ssl implementations concurrently])
+	])
+])
+AM_CONDITIONAL([MULTI_SSL], [test "x$enable_multi_ssl" = "xyes"])
+
+AS_IF([test "x$with_openssl" = "xyes"], [
+	PKG_CHECK_MODULES([OPENSSL], [openssl >= 0.9.8], [found_openssl=yes])
+])
+AM_CONDITIONAL([HAVE_OPENSSL], [test "x$found_openssl" = "xyes"])
+
+AS_IF([test "x$with_gnutls" = "xyes"], [
+	PKG_CHECK_MODULES([GNUTLS], [gnutls >= 3.5.8], [found_gnutls=yes])
+])
+AM_CONDITIONAL([HAVE_GNUTLS], [test "x$found_gnutls" = "xyes"])
+
+AS_IF([test "x$found_openssl" != "xyes" -a "x$found_gnutls" != "xyes"], [
+	AC_MSG_ERROR([Neither openssl nor gnutls was found])
 ])
 
+dnl work out default, or only, ssl library
+AC_MSG_CHECKING([which ssl library to use as default])
+AC_ARG_WITH([default-ssl], AS_HELP_STRING([--with-default-ssl@<:@=Lib@:>@],
+		[set default ssl library [openssl|gnutls] for multi-ssl]),
+	[with_default_ssl=$withval],
+	[with_default_ssl=yes])
+dnl note, yes/blank case will require further nesting, if additional ssl lib support added
+AS_CASE([$with_default_ssl],
+	[openssl], [AS_IF([test "x$found_openssl" = "xyes"], [ssl_impl=openssl],
+		[AC_MSG_ERROR([requested default ssl lib openssl, but openssl not found])]
+			)],
+	[gnutls], [AS_IF([test "x$found_gnutls" = "xyes"], [ssl_impl=gnutls],
+		[AC_MSG_ERROR([requested default ssl lib gnutls, but gnutls not found])]
+			)],
+	[no], [AC_MSG_ERROR([unsupported configuration --without-default-ssl])],
+	[yes], [AS_IF([test "x$found_openssl" = "xyes"], [ssl_impl=openssl],
+			[ssl_impl=gnutls])],
+	[AC_MSG_ERROR([Unknown ssl library $with_default_ssl])]
+)
+AC_MSG_RESULT([$ssl_impl])
+
 AC_ARG_WITH([examples],
 	AS_HELP_STRING([--without-examples], [do not build example programs]),
-	[],
+	[with_examples=$withval],
 	[with_examples=yes])
 
 AM_CONDITIONAL([BUILD_EXAMPLES], [test "x$with_examples" != "xno"])
 
+dnl should not reach default case, bail out if it triggers
+AS_CASE([$ssl_impl],
+	[openssl], [
+		AC_SUBST([SSL_DEF_LDFLAGS], [$OPENSSL_LDFLAGS])
+		AC_SUBST([SSL_DEF_INCLUDES], [$OPENSSL_INCLUDES])
+		AC_SUBST([SSL_DEF_CFLAGS], [$OPENSSL_CFLAGS])
+		AC_SUBST([SSL_DEF_LIBS], [$OPENSSL_LIBS])
+		],
+	[gnutls], [
+		AC_SUBST([SSL_DEF_LDFLAGS], [$GNUTLS_LDFLAGS])
+		AC_SUBST([SSL_DEF_INCLUDES], [$GNUTLS_INCLUDES])
+		AC_SUBST([SSL_DEF_CFLAGS], [$GNUTLS_CFLAGS])
+		AC_SUBST([SSL_DEF_LIBS], [$GNUTLS_LIBS])
+		],
+	[AC_MSG_ERROR([unable to set default ssl library])]
+)
+
 PKG_CHECK_MODULES([JANSSON], [jansson >= 2.0])
 PKG_CHECK_MODULES([CHECK], [check >= 0.9.4], [true], [true])
 
@@ -46,6 +120,13 @@ DX_PDF_FEATURE(OFF)
 DX_PS_FEATURE(OFF)
 DX_INIT_DOXYGEN($PACKAGE_NAME, doxygen.cfg)
 
+
+AM_COND_IF([MULTI_SSL], [
+	AM_COND_IF([HAVE_OPENSSL],
+		[AC_CONFIG_FILES([libjwt/libjwt-ossl.pc])])
+	AM_COND_IF([HAVE_GNUTLS],
+		[AC_CONFIG_FILES([libjwt/libjwt-gnutls.pc])])
+	])
 AC_CONFIG_FILES([
 	Makefile
 	include/Makefile
diff --git a/libjwt/Makefile.am b/libjwt/Makefile.am
index 9600317..c69b424 100644
--- a/libjwt/Makefile.am
+++ b/libjwt/Makefile.am
@@ -8,11 +8,44 @@ else
 libjwt_la_SOURCES += jwt-gnutls.c
 endif
 
+if MULTI_SSL
+if HAVE_OPENSSL
+lib_LTLIBRARIES += libjwt-ossl.la
+libjwt_ossl_la_SOURCES = jwt.c base64.c jwt-openssl.c
+endif
+if HAVE_GNUTLS
+lib_LTLIBRARIES += libjwt-gnutls.la
+libjwt_gnutls_la_SOURCES = jwt.c base64.c jwt-gnutls.c
+endif
+endif
+
 # http://www.gnu.org/software/libtool/manual/html_node/Updating-version-info.html
-libjwt_la_LDFLAGS = -version-info 9:1:8 $(OPENSSL_LDFLAGS) $(GNUTLS_LDFLAGS) $(JANSSON_LDFLAGS) -no-undefined
-libjwt_la_CPPFLAGS = -I$(top_srcdir)/include $(OPENSSL_INCLUDES) $(GNUTLS_INCLUDES) $(CODE_COVERAGE_CPPFLAGS) -Wall
-libjwt_la_CFLAGS = $(JANSSON_CFLAGS) $(OPENSSL_CFLAGS) $(GNUTLS_CFLAGS) $(CODE_COVERAGE_CFLAGS) -D_GNU_SOURCE
-libjwt_la_LIBADD = $(JANSSON_LIBS) $(OPENSSL_LIBS) $(GNUTLS_LIBS) $(CODE_COVERAGE_LDFLAGS)
+libjwt_la_LDFLAGS = -version-info 9:1:8 $(SSL_DEF_LDFLAGS) $(JANSSON_LDFLAGS) -no-undefined
+libjwt_la_CPPFLAGS = -I$(top_srcdir)/include $(SSL_DEF_INCLUDES) $(CODE_COVERAGE_CPPFLAGS) -Wall
+libjwt_la_CFLAGS = $(JANSSON_CFLAGS) $(SSL_DEF_CFLAGS) $(CODE_COVERAGE_CFLAGS) -D_GNU_SOURCE
+libjwt_la_LIBADD = $(JANSSON_LIBS) $(SSL_DEF_LIBS) $(CODE_COVERAGE_LDFLAGS)
+if MULTI_SSL
+if HAVE_OPENSSL
+libjwt_ossl_la_LDFLAGS = -version-info 9:1:8 $(OPENSSL_LDFLAGS) $(JANSSON_LDFLAGS) -no-undefined
+libjwt_ossl_la_CPPFLAGS = -I$(top_srcdir)/include $(OPENSSL_INCLUDES) $(CODE_COVERAGE_CPPFLAGS) -Wall
+libjwt_ossl_la_CFLAGS = $(JANSSON_CFLAGS) $(OPENSSL_CFLAGS) $(CODE_COVERAGE_CFLAGS) -D_GNU_SOURCE
+libjwt_ossl_la_LIBADD = $(JANSSON_LIBS) $(OPENSSL_LIBS) $(CODE_COVERAGE_LDFLAGS)
+endif
+if HAVE_GNUTLS
+libjwt_gnutls_la_LDFLAGS = -version-info 9:1:8 $(GNUTLS_LDFLAGS) $(JANSSON_LDFLAGS) -no-undefined
+libjwt_gnutls_la_CPPFLAGS = -I$(top_srcdir)/include $(GNUTLS_INCLUDES) $(CODE_COVERAGE_CPPFLAGS) -Wall
+libjwt_gnutls_la_CFLAGS = $(JANSSON_CFLAGS) $(GNUTLS_CFLAGS) $(CODE_COVERAGE_CFLAGS) -D_GNU_SOURCE
+libjwt_gnutls_la_LIBADD = $(JANSSON_LIBS) $(GNUTLS_LIBS) $(CODE_COVERAGE_LDFLAGS)
+endif
+endif
 
 pkgconfiglibdir = $(libdir)/pkgconfig
 pkgconfiglib_DATA = libjwt.pc
+if MULTI_SSL
+if HAVE_OPENSSL
+pkgconfiglib_DATA += libjwt-ossl.pc
+endif
+if HAVE_GNUTLS
+pkgconfiglib_DATA += libjwt-gnutls.pc
+endif
+endif
diff --git a/libjwt/libjwt-gnutls.pc.in b/libjwt/libjwt-gnutls.pc.in
new file mode 100644
index 0000000..63962ee
--- /dev/null
+++ b/libjwt/libjwt-gnutls.pc.in
@@ -0,0 +1,12 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: @PACKAGE_NAME@
+Description: JWT C Library
+Version: @PACKAGE_VERSION@
+
+Cflags: -I${includedir}
+Libs: -L${libdir} -ljwt-gnutls
+Libs.private: @JANSSON_LIBS@ @GNUTLS_LIBS@
diff --git a/libjwt/libjwt-ossl.pc.in b/libjwt/libjwt-ossl.pc.in
new file mode 100644
index 0000000..98c0ebe
--- /dev/null
+++ b/libjwt/libjwt-ossl.pc.in
@@ -0,0 +1,12 @@
+prefix=@prefix@
+exec_prefix=@exec_prefix@
+libdir=@libdir@
+includedir=@includedir@
+
+Name: @PACKAGE_NAME@
+Description: JWT C Library
+Version: @PACKAGE_VERSION@
+
+Cflags: -I${includedir}
+Libs: -L${libdir} -ljwt-ossl
+Libs.private: @JANSSON_LIBS@ @OPENSSL_LIBS@
diff --git a/libjwt/libjwt.pc.in b/libjwt/libjwt.pc.in
index 0c81d36..0bb64f4 100644
--- a/libjwt/libjwt.pc.in
+++ b/libjwt/libjwt.pc.in
@@ -9,4 +9,4 @@ Version: @PACKAGE_VERSION@
 
 Cflags: -I${includedir}
 Libs: -L${libdir} -ljwt
-Libs.private: @JANSSON_LIBS@ @OPENSSL_LIBS@
+Libs.private: @JANSSON_LIBS@ @SSL_DEF_LIBS@