Fix Linking for MinGW Libraries on *-windows-gnu Targets

[camconn]

When building for *-windows-gnu targets, link against .dll.a import libraries instead of .dll files directly. LLD doesn't support linking against DLLs, but requires you to link against a .lib or .dll.a instead.

Fixes #7799.

[andrewrk]

Thanks for the PR. I have a different plan for this: #6565

On windows zig will only deal with .lib files, not .dll.a files.

[camconn]

Thanks Andrew,

The issue with using .lib for linking external system libraries is that Mingw uses the (.dll)?.a extensions which e.g. cause #7799 and breaks Zig's pkg-config integration on Mingw. Mingw doesn't ship with .lib files, but rather ships static .a or dynamic import .dll.a libraries. AFAICT these are the same as .lib with a different file extension. If I'm wrong on any of that, please correct me.

This PR is not concerned with what Zig uses internally, so go on ahead with #6565. It's more concerned with compatibility for external packages and what Zig passes to lld for linking.

I've added a commit that should fix the CI build. Would you like any other changes, or should this PR just be closed?

[andrewrk]

If you don't mind, please leave it open, and when I implement #6565 I will have all the context loaded in my mind to know how the ideas in this PR should be merged (or not). Sound OK?

[camconn]

Yeah that's fine. It's your code, so ultimately your say.

In the meantime I've pushed added a patch that should fix builds for windows targets on non-windows OSs.

[KioriSun]

Hi all I tried compiling SDL2 on win with zig cc, and the current holdup is exactly that sdl ships with sdlLibs.a instead of .libs on the mingw devel.
I believe zig should just support the default for mingw.

[andrewrk]

Please see #7799 (comment) for how I am solving this.