add support for mingw build

[rofl0r]

this allows to easily cross-compile the windows gui app for 32 or 64 bits even.

[zeromus]

as long as it doesn't break msvc. let me know when youre done

[rofl0r]

should be complete from my side. you might want to try the aviout.cpp fix though; i'm not 100% sure it's correct, and avi recording crashes in the mingw build - though that might have other reasons. once it's merged i can set up a CI job

[rofl0r]

ok, both aviout and jit are fixed now. the jit issue was caused by mingw having its own STDCALL macro - see 18f166b . for avi i found the proper way to call another constructor via delegation.

[crashGG]

Hi,could you add mingw-based builds to the workflow?

[rofl0r]

@zeromus so - is it ok for you to merge this? since there's currently some refactor stuff going on i fear that merging this will soon become a pain, so i guess it's best to do it ASAP while everything still works.

[zeromus]

As long as it doesn't break msvc.. sure..