Make glx_static report skip if it was built non-static.

It won't have that symbol, unless it happens to be linked against the
full library.

test/glx_static.c

@@ -47,6 +47,11 @@ main(int argc, char **argv)
     bool pass = true;
     int val;
 
+    if (dlsym(NULL, "epoxy_glCompileShader")) {
+        fprintf(stderr, "glx_static requires epoxy built with --enable-static\n");
+        return 77;
+    }
+
     Display *dpy = get_display_or_skip();
     make_glx_context_current_or_skip(dpy);