I've tried to compile mupen64plus against MXE (mingw crosscompile environment) and found following problem with SDL2 in MXE:

* https://travis-ci.org/charlemagnelasse/mupen64plus-video-glide64/jobs/297566071
* https://travis-ci.org/charlemagnelasse/mupen64plus-video-glide64/jobs/297566072
* https://travis-ci.org/charlemagnelasse/mupen64plus-video-rice/jobs/297566188
* https://travis-ci.org/charlemagnelasse/mupen64plus-video-rice/jobs/297566189

Problem is that SDL_opengl.h is declaring prototypes even when GL_GLEXT_PROTOTYPES is not set. And when you set GL_GLEXT_PROTOTYPES then it declares it twice (one time in SDL_opengl.h and one time in SDL_opengl_glext.h):

> i686-w64-mingw32.shared-g++ -fvisibility-inlines-hidden -std=gnu++0x -O3 -flto -Wall -Wno-unused-function -ffast-math -fno-strict-aliasing -fvisibility=hidden -I../../src -I../../src/Glitch64/inc -DGCC -mmmx -msse -fno-PIC -I/usr/lib/mxe/usr/i686-w64-mingw32.shared/include   -I/usr/lib/mxe/usr/i686-w64-mingw32.shared/include/libpng16    -I/usr/lib/mxe/usr/i686-w64-mingw32.shared/include/SDL2 -Dmain=SDL_main -DGL_GLEXT_PROTOTYPES "-I/home/travis/build/charlemagnelasse/mupen64plus-video-glide64mk2/deps/mupen64plus-core/src/api/" -MD -MP -DNO_FILTER_THREAD -DTEXTURE_FILTER -DDUMP_CACHE  -c -o _obj/Glitch64/OGLgeometry.o ../../src/Glitch64/OGLgeometry.cpp
> In file included from ../../src/Glitch64/OGLcombiner.cpp:30:0:
> ../../src/Glitch64/glitchmain.h:77:38: error: 'void (__attribute__((__stdcall__)) * glActiveTextureARB)(GLenum)' redeclared as different kind of symbol
>      extern PFNGLACTIVETEXTUREARBPROC glActiveTextureARB;
>                                       ^
> In file included from ../../src/Glitch64/glitchmain.h:76:0,
>                  from ../../src/Glitch64/OGLcombiner.cpp:30:
> /usr/lib/mxe/usr/i686-w64-mingw32.shared/include/SDL2/SDL_opengl.h:2019:23: note: previous declaration 'void glActiveTextureARB(GLenum)'
>  GLAPI void GLAPIENTRY glActiveTextureARB(GLenum texture);
>                        ^
> In file included from ../../src/Glitch64/OGLcombiner.cpp:30:0:
> ../../src/Glitch64/glitchmain.h:78:37: error: 'void (__attribute__((__stdcall__)) * glAttachObjectARB)(GLhandleARB, GLhandleARB)' redeclared as different kind of symbol
>      extern PFNGLATTACHOBJECTARBPROC glAttachObjectARB;
>                                      ^
> In file included from /usr/lib/mxe/usr/i686-w64-mingw32.shared/include/SDL2/SDL_opengl.h:2098:0,
>                  from ../../src/Glitch64/glitchmain.h:76,
>                  from ../../src/Glitch64/OGLcombiner.cpp:30:
> /usr/lib/mxe/usr/i686-w64-mingw32.shared/include/SDL2/SDL_opengl_glext.h:3659:21: note: previous declaration 'void glAttachObjectARB(GLhandleARB, GLhandleARB)'
>  GLAPI void APIENTRY glAttachObjectARB (GLhandleARB containerObj, GLhandleARB obj);
>                      ^
> In file included from ../../src/Glitch64/OGLcombiner.cpp:30:0:
> ../../src/Glitch64/glitchmain.h:79:40: error: 'void (__attribute__((__stdcall__)) * glBindFramebufferEXT)(GLenum, GLuint)' redeclared as different kind of symbol
>      extern PFNGLBINDFRAMEBUFFEREXTPROC glBindFramebufferEXT;

The commit introducing this problem is http://hg.libsdl.org/SDL/file/c94127dcbe97/include/SDL_opengl.h . And this commit can be reverted to fix this problem.

Tried to use -DNO_SDL_GLEXT but then even more stuff is undefined (tested with both "-DNO_SDL_GLEXT" and "-DNO_SDL_GLEXT -DGL_GLEXT_PROTOTYPES"):

> /usr/lib/mxe/usr/i686-w64-mingw32.shared/include/SDL2/SDL_opengl.h:2031:23: note: previous declaration 'void glMultiTexCoord2fARB(GLenum, GLfloat, GLfloat)'
>  GLAPI void GLAPIENTRY glMultiTexCoord2fARB(GLenum target, GLfloat s, GLfloat t);
>                        ^
> ../../src/Glitch64/glitchmain.h:140:34: error: 'void (__attribute__((__stdcall__)) * glActiveTextureARB)(GLenum)' redeclared as different kind of symbol
>  extern PFNGLACTIVETEXTUREARBPROC glActiveTextureARB;
>                                   ^
> In file included from ../../src/Glitch64/glitchmain.h:76:0,
>                  from ../../src/Glitch64/OGLgeometry.cpp:26:
> /usr/lib/mxe/usr/i686-w64-mingw32.shared/include/SDL2/SDL_opengl.h:2019:23: note: previous declaration 'void glActiveTextureARB(GLenum)'
>  GLAPI void GLAPIENTRY glActiveTextureARB(GLenum texture);
>                        ^
> In file included from ../../src/Glitch64/OGLgeometry.cpp:26:0:
> ../../src/Glitch64/glitchmain.h:141:36: error: 'void (__attribute__((__stdcall__)) * glMultiTexCoord2fARB)(GLenum, GLfloat, GLfloat)' redeclared as different kind of symbol
>  extern PFNGLMULTITEXCOORD2FARBPROC glMultiTexCoord2fARB;
>                                     ^
> In file included from ../../src/Glitch64/glitchmain.h:76:0,
>                  from ../../src/Glitch64/OGLgeometry.cpp:26:
> /usr/lib/mxe/usr/i686-w64-mingw32.shared/include/SDL2/SDL_opengl.h:2031:23: note: previous declaration 'void glMultiTexCoord2fARB(GLenum, GLfloat, GLfloat)'
>  GLAPI void GLAPIENTRY glMultiTexCoord2fARB(GLenum target, GLfloat s, GLfloat t);
>                        ^
> ../../src/Glitch64/OGLgeometry.cpp: In function 'void grDrawTriangle(const void*, const void*, const void*)':
> ../../src/Glitch64/OGLgeometry.cpp:390:60: error: 'glSecondaryColor3f' was not declared in this scope
>        glSecondaryColor3f((1.0f / *a_q) / 255.0f, 0.0f, 0.0f);
>                                                             ^
> ../../src/Glitch64/OGLgeometry.cpp:392:62: error: 'glSecondaryColor3f' was not declared in this scope
>        glSecondaryColor3f((1.0f / *a_fog) / 255.0f, 0.0f, 0.0f);
                                                              ^

This problem is here since SDL 2.0.4. And users also reported this problem outside of MXE (with MSYS2 as shown in  https://github.com/mupen64plus/mupen64plus-video-glide64mk2/issues/90).