configure: use -Werror to make the include GL and GLES2 header check guaranteed fatal

Clang will only give a warning for the redefinition of typedef GLenum.
Since master is build with -Werror this will result in a build failure
later in the gl plugin. Add -Werror to the test, so the test result is as
expected. This will allow the gl plugins to build.

https://bugzilla.gnome.org//show_bug.cgi?id=731692

configure.ac

@@ -851,6 +851,8 @@ if test "x$HAVE_GL" = "xyes" -a "x$HAVE_GLES2" = "xyes"; then
 # endif
 "
   AC_MSG_CHECKING([whether it is possible to include both GL and GLES2 headers])
+  save_CPPFLAGS="$CPPFLAGS"
+  CPPFLAGS="$CPPFLAGS -Werror"
   AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[$GL_INCLUDES]], [[
     #if !defined(GL_FALSE)
     #error Failed to include GL headers
@@ -868,6 +870,7 @@ if test "x$HAVE_GL" = "xyes" -a "x$HAVE_GLES2" = "xyes"; then
       HAVE_GLES2=no
     fi
   ])
+  CPPFLAGS="$save_CPPFLAGS"
 fi
 
 #dnl Check for OpenGL, GLU