CMake: Enable plugins on MinGW

[MehdiChinoune]

They could be built on MinGW.

[CLAassistant]

All committers have signed the CLA.

[DWesl]

Are you planning to add CMake to the MinGW CI run?

You could borrow from the Linux CMake workflow, or from my most recent attempt to get CMake working on MinGW (compile fails).

[MehdiChinoune]

Are you planning to add CMake to the MinGW CI run?

You could borrow from the Linux CMake workflow, or from my most recent attempt to get CMake working on MinGW (compile fails).

I was planning, but I prefered to add it in a separate PR.

[DWesl]

That's a sensible concern

[MehdiChinoune]

That's a sensible concern

I don't understand?

[DWesl]

Trying to keep a PR focused on a single change, rather than allowing its scope to expand to include many tangentially-related things, is a sensible concern.

[WardF]

@DennisHeimbigner These changes largely look good to me, but I'm uncertain of the effect the change to WIN32 from MSVC will have in cygwin-based builds. You work in Cygwin much more frequently than I do; if Cygwin sets Win32 but uses Unix-style library names, this could cause a problem (I suspect). All of this is untested conjecture, and I can't easily test cygwin at the moment. Would you mind taking a look and approving these changes if they don't cause a problem?

[MehdiChinoune]

I think WIN32 is not set to true on CYGWIN by CMake.

[DWesl]

Cygwin CMake spent a few years telling me it no longer set _WIN32 on Cygwin, so I suspect it's a separate thing.

A small CMakeLists.txt set to output messages if WIN32 or CYGWIN is defined outputs the Cygwin message but not the Windows one.

[DennisHeimbigner]

Are we talking about "WIN32" or "_WIN32"? The latter is set by the compiler.
As far as I know, we never use "WIN32" anywhere in our code; we only use "_WIN32".
Also, specifying MSVC is sometimes the correct test (vs _WIN32) when the issue is
building on windows using gcc or clang vs building using the visual studio compiler.

[MehdiChinoune]

We are talking about WIN32 in CMake language.