Do not require a particular SVG library unless actually found.

--- a/configure.ac	2015-05-11 00:13:51.180002639 +0200
+++ b/configure.ac	2015-05-11 00:13:40.365069049 +0200
@@ -319,7 +319,9 @@
 
   PKG_CHECK_EXISTS([librsvg-2.0 >= $RSVG_REQUIRED],[have_rsvg=yes],[have_rsvg=no])
 
-  PKGS="$PKGS librsvg-2.0 >= $RSVG_REQUIRED cairo >= $CAIRO_REQUIRED"
+  if test "$have_rsvg" = "yes" ; then
+    PKGS="$PKGS librsvg-2.0 >= $RSVG_REQUIRED cairo >= $CAIRO_REQUIRED"
+  fi
 
   AC_DEFINE([HAVE_RSVG],[1],[Refine if librsvg is available])
 fi
@@ -334,8 +336,10 @@
 
   PKG_CHECK_EXISTS([QtSvg >= $QT_SVG_REQUIRED],[have_qtsvg=yes],[have_qtsvg=no])
 
-  PKGS="$PKGS QtSvg >= $QT_SVG_REQUIRED"
-  EXTRA_CXX_LIBS="$EXTRA_CXX_LIBS -lstdc++"
+  if test "$have_qtsvg" = "yes" ; then
+    PKGS="$PKGS QtSvg >= $QT_SVG_REQUIRED"
+    EXTRA_CXX_LIBS="$EXTRA_CXX_LIBS -lstdc++"
+  fi
 
   AC_DEFINE([HAVE_QTSVG],[1],[Refine if QtSvg is available])
 fi