Made it an error to not define an entry point retrieval mechanism on X11.

src/opengl.c

@@ -411,14 +411,8 @@ GLboolean _glfwIsValidContext(_GLFWwindow* window, _GLFWwndconfig* wndconfig)
         window->GetStringi = (PFNGLGETSTRINGIPROC) glfwGetProcAddress("glGetStringi");
         if (!window->GetStringi)
         {
-            // This is a very common problem among people who compile GLFW
-            // on X11/GLX using custom build systems, as it needs explicit
-            // configuration in order to work
-
             _glfwSetError(GLFW_PLATFORM_ERROR,
-                          "glfwOpenWindow: Entry point retrieval is broken "
+                          "glfwOpenWindow: Entry point retrieval is broken");
-                          "because GLFW was incorrectly compiled; "
-                          "see the build documentation for your platform");
             return GL_FALSE;
         }
     }

src/x11_opengl.c

@@ -48,7 +48,7 @@ void (*glXGetProcAddressEXT(const GLubyte* procName))();
  #define _glfw_glXGetProcAddress(x) dlsym(_glfwLibrary.X11.libGL, x)
  #define _GLFW_DLOPEN_LIBGL
 #else
-#define _glfw_glXGetProcAddress(x) NULL
+ #error "No OpenGL entry point retrieval mechanism was enabled"
 #endif