opengl32: it does not compile well in CYGWIN and MINGW

[carlo.bramix at libero.it]

Hello,
I'm writing here for asking a suggestion about a patch that I have sent one month ago.
The fix attached in this email:

https://www.winehq.org/pipermail/wine-patches/2015-August/141811.html

is required because, when compiling opengl32 on Windows (CYGWIN or MINGW), the compiler complains that the exported functions are redeclared without dllimport attribute, while they should be declared with dllexport from beginning.
This error can be fixed by adding the definition of _GDI32_ so that WINGDIAPI macro is declared correctly.
This fix already exists for example in /dlls/gdi32/Makefile.in as you can see here:

http://source.winehq.org/git/wine.git/blob/HEAD:/dlls/gdi32/Makefile.in

I hope that it could be committed because perhaps it does not make difference on platforms without dllexport/dllimport attribute support, but it does on real Windows.

Thank you.

Sincerely,

Carlo