[sumo-user] fix for SUMO mac build

[Jad Nohra <jad.nohra@xxxxxxxxxxxxxx>]

Hello,

I am trying to build SUMO on macOS 10.13.4, it has clang installed through XCode.

I used a couple of the tips mentioned in 

https://sourceforge.net/p/sumo/mailman/message/32092145/

But with the modifications:

1. The only file that now has to be modified to fixed OpenGL compilation errors is: http://./src/utils/gui/globjects/GLIncludes.h
2. With the current clang (Apple LLVM version 9.1.0 (clang-902.0.39.2), I get an error 

   1. GLHelper.cpp:56:6: error: variable has incomplete type 'void'

      void APIENTRY combCallback(GLdouble coords[3],

           ^

      GLHelper.cpp:56:14: error: expected ';' after top level declarator

      void APIENTRY combCallback(GLdouble coords[3],

                   ^

                   ;

This is due to APIENTRY only being defined on windows, or in glew.h (which SUMO does not use). 

My fix was again to edit GLIncludes.h, so both fixes together amount to:

#ifdef __APPLE__

#  define APIENTRY

#  include <OpenGL/gl.h>

#  include <OpenGL/glu.h>

#else

#  include <GL/gl.h>

#  include <GL/glu.h>

#endif

My question is, does it make sense to create a pull request with this fix?

Thanks!

Jad