Add support for MinGW

[rasky]

This PR adds support for compilation with MinGW, and modifies the CI to also run MinGW in the test matrix to avoid regressions.

Modifications are trivial and listed here:

• _MSC_VER was used to both test for MSVC and Windows. Changed it to _WIN32 where it actually meant "check for Windows" (eg: around all usages of Windows APIs).
• To compile MS structure definitions that use non-standard C++ code (anonymous structs in unions), it is necessary to either disable -Wpedantic or use -fms-extensions. It is my understanding that subprocess.h tries to be compatible with the strictest compile time options, so I went for scoped disabling of -Wpedantic via #pragma
• In MinGW, weak functions are basically non-working because of weird interactions with PE/COFF. I've changed the definition of subprocess_weak to static __attribute__((used)) for MinGW. Yes, it means the code gets linked multiple times, but I don't know of any other solution. Other single-file libraries opt for an external define to differentiate declaration vs definition, but since that's not the case here, it looks like duplicating code is the only solution.
• I also did some similar changes to utest.h. I understand that utest.h is separately maintained, but I guess it can be upstreamed later if this PR is approved.
• MSVC predefines _DLL as an empty macro instead of giving it value 1 like MSVC does. I've changed the check to cover both cases.

Some tests that are disabled for MSVC appears to pass in MinGW. Activate those.

[rasky]

I forgot to mention two unrelated changes that I’ve left in, but I can back out if you prefer:

• Change CI to run in all branches. This was useful to me to test the changes in my fork before opening the PR. I can back it out if you feel strongly, but I think it makes it harder for contributors to test modifications.
• clang-10 doesn’t exist anymore in today’s ubuntu-latest image, so CI is actually broken right now on master. I changed it to be a rolling package “clang”, but let me know what you prefer here.