amend! mingw: rely on MSYS2's metadata instead of hard-coding it

mingw: rely on MSYS2's metadata instead of hard-coding it

MSYS2 defines some helpful environment variables, e.g. `MSYSTEM`. There
is code in Git for Windows to ensure that that `MSYSTEM` variable is
set, hard-coding a default.

However, the existing solution jumps through hoops to reconstruct the
proper default, and is even incomplete doing so, as we found out when we
extended it to support CLANGARM64.

This is absolutely unnecessary because there is already a perfectly
valid `MSYSTEM` value we can use at build time. This is even true when
building the MINGW32 variant on a MINGW64 system because `makepkg-mingw`
will override the `MSYSTEM` value as per the `MINGW_ARCH` array.

The same is equally true for the `/mingw64`, `/mingw32` and
`/clangarm64` prefix: those values are already available via the
`MINGW_PREFIX` environment variable, and we just need to pass that
setting through.

Only when `MINGW_PREFIX` is not set (as is the case in Git for Windows'
minimal SDK, where only `MSYSTEM` is guaranteed to be set correctly), we
use as fall-back the top-level directory whose name is the down-cased
value of the `MSYSTEM` variable.

Incidentally, this also broadens the support to all the configurations
supported by the MSYS2 project, i.e. clang64 & ucrt64, too.

Unfortunately, this incidentally _narrows_ the support when building
with CMake or Meson, but this change's benefit outweighs this downside.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

contrib/buildsystems/CMakeLists.txt

@@ -283,6 +283,11 @@ if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
 				NOGDI OBJECT_CREATION_MODE=1 __USE_MINGW_ANSI_STDIO=0
 				USE_NED_ALLOCATOR OVERRIDE_STRDUP MMAP_PREVENTS_DELETE USE_WIN32_MMAP
 				HAVE_WPGMPTR HAVE_RTLGENRANDOM)
+	if(CMAKE_SYSTEM_PROCESSOR STREQUAL "x64")
+		add_compile_definitions(ENSURE_MSYSTEM_IS_SET="MINGW64" MINGW_PREFIX="/mingw64")
+	elseif(CMAKE_SYSTEM_PROCESSOR STREQUAL "arm64")
+		add_compile_definitions(ENSURE_MSYSTEM_IS_SET="CLANGARM64" MINGW_PREFIX="/clangarm64")
+	endif()
 	list(APPEND compat_SOURCES
 		compat/mingw.c
 		compat/winansi.c