wgl: Remove opengl32.mingw.def.
MinGW DEF parsing is even more broken than before, and 32-bits import libs are broken, regardless one uses opengl32.mingw.def or opengl32.def.
This change removes opengl32.mingw.def and addresses the issue differently:
- link opengl32.dll with --enable-stdcall-fixup
- use the systems opengl32 import lib (libopengl32.a/opengl32.lib) instead of our own
This change also gets test_wgl built with MinGW (even if it's never tested), which I used to verify this; and to not link against internal libraries, which seemed confusing and unnecessary.