If glvnd library found, do not use libGL.so in epoxy_load_gl

Without additional check, even if libOpenGL was loaded, libGL.so will
be loaded as well, and used both in gl_handle and glx_handle, so
libglvnd libraries will not be used.

Reviewed-by: Adam Jackson <ajax@redhat.com>

Signed-off-by: Emmanuele Bassi <ebassi@gnome.org>

src/dispatch_common.c

@@ -674,9 +674,12 @@ epoxy_load_gl(void)
     if (!api.gl_handle)
 	get_dlopen_handle(&api.gl_handle, OPENGL_LIB, false, true);
 #endif
-
-    get_dlopen_handle(&api.glx_handle, GLX_LIB, true, true);
-    api.gl_handle = api.glx_handle;
+    if (!api.gl_handle) {
+	get_dlopen_handle(&api.gl_handle, GLX_LIB, true, true);
+#if PLATFORM_HAS_GLX
+	api.glx_handle = api.gl_handle;
+#endif
+    }
 #endif
 }