gh-90815: Define WITH_MIMALLOC macro in PC/pyconfig.h

[vstinner]

• Issue: Add mimalloc memory allocator #90815

[vstinner]

Example of a recent PR, test.pythoninfo of Windows x64 says:

build.WITH_MIMALLOC: No 

[vstinner]

The build fails on Windows. Example:

obmalloc.obj : error LNK2005: _mi_free already defined in alloc.obj [D:\a\cpython\cpython\PCbuild\_freeze_module.vcxproj] 

cc @DinoV

[DinoV]

The build fails on Windows. Example:

obmalloc.obj : error LNK2005: _mi_free already defined in alloc.obj [D:\a\cpython\cpython\PCbuild\_freeze_module.vcxproj] 

cc @DinoV

I think the issue here is that we're including the files in the Windows build but they're now included via obmalloc.c (this had changed along the way). I'll let the build bots see if #111532 fixes it.

[vstinner]

@DinoV wrote the correct fix: #111532