The UCRT MinGW flavor

[slipher]

At some point we will probably need to support building with MinGW toolchains with the UCRT runtime flavor. (This is as opposed to MSVCRT; see wiki for more information.) The obstacle now is that the external_deps libs link against MSVCRT-specific symbols. The MSVCRT toolchains are being deprecated or removed by some distributions:

• In MSYS2, the MSVCRT toolchains are deprecated. But then, all 32-bit target toolchains are also deprecated.
• In Arch Linux, MSVCRT toolchains are removed. 32- and 64-bit UCRT toolchains are provided.

In Debian stable, there is only a 64-bit UCRT toolchain provided, in addition to 32- and 64-bit MSVCRT ones.

Maybe it would be possible to tweak the deps build script so that the libraries are compatible with both UCRT and MSVCRT flavors, like we did for making them compatible with different exception flavors.

[slipher]

You could probably build with a UCRT toolchain right now using one of these methods:

• Install deps with a package manager instead of using external_deps.
• (Maybe) use the MSVC external_deps instead of the MinGW one. This would probably work since the MSVC builds use DLLs for everything and those can work with any compiler, and GCC knows how to link to a DLL without an import lib.

Either of these methods could also be applied to build with the Clang which imitates MinGW, since it couldn't consume some of the MinGW-built static libraries last time I tried.