Fix GLU header inclusion being disabled

This fixes a regression introduced by
105cf32e0b where GLFW_INCLUDE_GLU had no
effect if the GLFW header detected an earlier OpenGL header or loader
library header.

Closes #1712.

include/GLFW/glfw3.h

@@ -193,6 +193,22 @@ extern "C" {
 
  #endif /*__APPLE__*/
 
+#elif defined(GLFW_INCLUDE_GLU)
+
+ #if defined(__APPLE__)
+
+  #if defined(GLFW_INCLUDE_GLU)
+   #include <OpenGL/glu.h>
+  #endif
+
+ #else /*__APPLE__*/
+
+  #if defined(GLFW_INCLUDE_GLU)
+   #include <GL/glu.h>
+  #endif
+
+ #endif /*__APPLE__*/
+
 #elif !defined(GLFW_INCLUDE_NONE) && \
       !defined(__gl_h_) && \
       !defined(__gles1_gl_h_) && \
@@ -216,9 +232,6 @@ extern "C" {
    #define GL_GLEXT_LEGACY
   #endif
   #include <OpenGL/gl.h>
-  #if defined(GLFW_INCLUDE_GLU)
-   #include <OpenGL/glu.h>
-  #endif
 
  #else /*__APPLE__*/
 
@@ -226,9 +239,6 @@ extern "C" {
   #if defined(GLFW_INCLUDE_GLEXT)
    #include <GL/glext.h>
   #endif
-  #if defined(GLFW_INCLUDE_GLU)
-   #include <GL/glu.h>
-  #endif
 
  #endif /*__APPLE__*/