AppleGLGetProcAddress: 820 warnings

[miguelmartin75]

There are 820 warnings for Mac builds due to the AppleGLGetProcAddress's signature. Here is a sample of the warnings:

/Users/miguel/Documents/Programming/Projects/Repos/magnum/external/OpenGL/GL/gl_magnum.c:2711:86: warning:
  passing 'char [12]' to parameter of type 'const GLubyte *' (aka
  'const unsigned char *') converts between pointers to integer types with
  different sign [-Wpointer-sign]
...GLubyte * (CODEGEN_FUNCPTR *)(GLenum))IntGetProcAddress("glGetString");
                                                         ^~~~~~~~~~~~~
/Users/miguel/Documents/Programming/Projects/Repos/magnum/external/OpenGL/GL/gl_magnum.c:86:57: note:
  expanded from macro 'IntGetProcAddress'
            #define IntGetProcAddress(name) AppleGLGetProcAddress(name)
                                                                  ^
/Users/miguel/Documents/Programming/Projects/Repos/magnum/external/OpenGL/GL/gl_magnum.c:9:52: note:
  passing argument to parameter 'name' here
static void* AppleGLGetProcAddress (const GLubyte *name)

[mosra]

Thanks for the detailed report and pull request. This issue is actually related to glLoadGen, which was used to generate gl_magnum.c file. I found a possible solution at https://bitbucket.org/alfonse/glloadgen/issue/29/compilation-errors-on-mac-os-x and added the change in ebe691e. It fixes this issue and should also fix #21, hopefully it won't cause any regressions. Please test and report any problems.