MSVC: support building as static library

[tobiasdiez]

Adds support for building flint as a static library using cmake.

Workaround for #2164 and might be the preferred option anyway in certain settings.

[albinahlback]

Sorry, but #2164 was caused because of some unsupported way of building FLINT?

So if you set BUILD_SHARED_LIBS to off when configuring CMake, it still builds a shared library?

[albinahlback]

To reiterate this, we do not maintain CMake to be perfect. Our CMake should work for Windows systems, but not more than that. It is not a replacement for our Autotools build system.

[tobiasdiez]

So if you set BUILD_SHARED_LIBS to off when configuring CMake, it still builds a shared library?

No it builds a static library, which is actually the default, see https://cmake.org/cmake/help/latest/variable/BUILD_SHARED_LIBS.html#variable:BUILD_SHARED_LIBS.

[tobiasdiez]

Our CMake should work for Windows systems, but not more than that.

I'm only using it for Windows to build flint before building sage as part of sagemath/sage#38872. The currently released version of flint has to many bugs on Windows to work but thanks to your recent bug fixes the unreleased version is working fine, at least with these changes here and #2156 (comment).

[albinahlback]

I'm confused. If building FLINT as a static library in CMake already works, why this PR?

[tobiasdiez]

It works with this PR applied...

[albinahlback]

Okay, I suppose I get confused by the wording. This does not add support for static library builds, but rather fixes it?

[isuruf]

This is also incomplete as downstream projects would have to add FLINT_STATIC_BUILD. #2170 is better