Make X11 detection more precise

Don't be content with just X11/Xlib.h, check for X11/XKBlib.h as well.
This prevents false positives (for example, from partial X11 headers
installed by tcl/tk).

https://bugzilla.gnome.org/show_bug.cgi?id=729513

configure.ac

@@ -541,7 +541,9 @@ AG_GST_CHECK_FEATURE(X, [X libraries and plugins],
   CPPFLAGS="$CPPFLAGS $X_CFLAGS"
 
   dnl now try to find the HEADER
-  AC_CHECK_HEADER([X11/Xlib.h], [HAVE_X="yes"], [HAVE_X="no"], [AC_INCLUDES_DEFAULT])
+  HAVE_X="yes"
+  AC_CHECK_HEADER([X11/Xlib.h], [], [HAVE_X="no"], [AC_INCLUDES_DEFAULT])
+  AC_CHECK_HEADER([X11/XKBlib.h], [], [HAVE_X="no"], [AC_INCLUDES_DEFAULT])
 
   if test "x$HAVE_X" = "xno"
   then