added warning if another GL loader or header was included before our loaders are included

include/ACGL/OpenGL/GL.hh

@@ -174,8 +174,9 @@
         #else
             // use the internal loader:
             #define ACGL_EXTENSION_LOADER_GLLOADGEN
-            // prevent other GL headers from getting included and redefine GL:
-            #define __gl3_h_
+            #if defined(__gl_h_) || defined(__GL_H__) || defined(__glext_h_) || defined(__GLEXT_H_) || defined(__gl_ATI_h_) || defined(__gl3_h_)
+            #error ACGL/GL.hh has to be the first OpenGL related file to include!
+            #endif
 
             //
             // Include the right header which has just what is needed to catch compatibility problems at compiletime.
@@ -217,6 +218,8 @@
                     #include <ACGL/OpenGL/glloaders/gl_compatibility_42.hh>
                 #endif
             #endif // ACGL_OPENGL_PROFILE_CORE
+            // prevent other GL headers from getting included and redefine GL:
+            #define __gl3_h_
         #endif // ACGL_USE_GLEW
     #else
         #define ACGL_USE_GLEW