Allow libopengl.so to be used when GLX_LIB is missing

This maintains compatibility with previous behavior of
always using GLX_LIB if it is found. The only change is
when there is no GLX_LIB.

Previous behavior when no GLX_LIB:
- abort.

New behavior when no GLX_LIB:
- Try to load libOpenGL.so as gl_handle (glx_handle remains NULL).
- Else, abort.

src/dispatch_common.c

@@ -670,13 +670,20 @@ epoxy_load_gl(void)
     get_dlopen_handle(&api.gl_handle, OPENGL_LIB, true, true);
 #else
 
+    // Prefer GLX_LIB over OPENGL_LIB to maintain existing behavior.
+    get_dlopen_handle(&api.glx_handle, GLX_LIB, false, true);
+    api.gl_handle = api.glx_handle;
+
 #if defined(OPENGL_LIB)
     if (!api.gl_handle)
-	get_dlopen_handle(&api.gl_handle, OPENGL_LIB, false, true);
+        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) {
+        fprintf(stderr, "Couldn't open %s or %s\n", GLX_LIB, OPENGL_LIB);
+        abort();
+    }
+
 #endif
 }