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

Commit dbfa4b20 has introduced a string of regressions in the X server
and KWin.

This reverts commit dbfa4b209c.

See: #252

src/dispatch_common.c

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