GLX: Add support for loading glvnd libGLX

src/glx_context.c

@@ -226,7 +226,10 @@ static GLFWglproc getProcAddressGLX(const char* procname)
     else if (_glfw.glx.GetProcAddressARB)
         return _glfw.glx.GetProcAddressARB((const GLubyte*) procname);
     else
+    {
+        // NOTE: glvnd provides GLX 1.4, so this can only happen with libGL
         return _glfwPlatformGetModuleSymbol(_glfw.glx.handle, procname);
+    }
 }
 
 static void destroyContextGLX(_GLFWwindow* window)
@@ -262,6 +265,7 @@ GLFWbool _glfwInitGLX(void)
 #elif defined(__OpenBSD__) || defined(__NetBSD__)
         "libGL.so",
 #else
+        "libGLX.so.0",
         "libGL.so.1",
         "libGL.so",
 #endif