Adds a new configure option to remove the automagic dependency on libX11.

--- configure.ac
+++ configure.ac
@@ -929,7 +929,18 @@ GDK_PIXBUF_XLIB_PACKAGES=
 GDK_PIXBUF_XLIB_EXTRA_CFLAGS=
 GDK_PIXBUF_XLIB_EXTRA_LIBS=
 
-if $PKG_CONFIG --exists x11; then
+AC_ARG_WITH(x11,
+            AC_HELP_STRING([--with-x11=@<:@no/auto/yes@:>@],
+                           [build X11 support @<:@default=auto@:>@]),,
+            [
+              if $PKG_CONFIG --exists x11; then
+                with_x11=yes
+              else
+                with_x11=no
+              fi
+            ])
+
+if test x$with_x11 = xyes; then
   AM_CONDITIONAL(USE_X11, true)
   PKG_CHECK_MODULES(GDK_PIXBUF_XLIB_DEP, x11 gobject-2.0)
 else