CheckTypeSize fails when running in MSYS2

[jdpipe]

Describe the bug
The following fairly trivial SConstruct file fails on MSYS2:

# SConstuct
import os
env = Environment(tools=["default","mingw"])
conf = Configure(env)
conf.env['SIZEOF_INT'] = conf.CheckTypeSize("int")
conf.env['SIZEOF_VOIDP'] = conf.CheckTypeSize("void *")
env = conf.Finish()
print("size of int:",env['SIZEOF_INT'])
print("size of void*:",env['SIZEOF_VOIDP'])
print("env['PLATFORM'] =",env['PLATFORM'])
print("env['HOST_OS'] =",env['HOST_OS'])

env.Program('hello',['hello.c'])

// hello.c
#include<stdio.h>
int main(void){fprintf(stderr,"hello from C\n"); return 0;}

The output is below. The output '.sconf_temp' is not recognized comes from Windows being passed a forward-slash-separated path, because on this version of Python, os.path.sep is set to /.

john@WIN-HG6RBDGB9LV UCRT64 ~/testsize
$ scons --config=force
scons: Reading SConscript files ...
'.sconf_temp' is not recognized as an internal or external command,
operable program or batch file.
Checking size of int ... no
'.sconf_temp' is not recognized as an internal or external command,
operable program or batch file.
Checking size of void * ... no
size of int: 0
size of void*: 0
env['PLATFORM'] = win32
env['HOST_OS'] = win32
scons: done reading SConscript files.
scons: Building targets ...
scons: `.' is up to date.
scons: done building targets.

A solution for this issue would be the automatic detection of the MSYS platform, (issue #4311), but there may be other simpler fixes to consider.

Required information

• Discord discussion: https://discord.com/channels/571796279483564041/1078868924248105032
• Version of SCons: 4.5.1
• Version of Python: 3.10.10 (mingw-w64-ucrt-x86_64-python 3.10.10-1 in MSYS2)
• Which python distribution if applicable: MSYS2
• How you installed SCons: pip install scons
• What Platform are you on? MSY2
• How to reproduce your issue? As above.
• How you invoke scons scons --config=force

[jdpipe]

The workaround for this issue is to invoke MSYSTEM= scons, which disables MSYS2's Python from setting os.path.sep to /. However, this is inconvenient and undesirable for other reasons, as a 'general solution'.

[mwichmann]

A lot of the discussion around the questions opened by this ended up discussed elsewhere, but this particular issue should actually have been resolved by PR #4307.