CCompiler._make_relative ineffective with Python 3.13

[mglisse]

(This was noticed in pypa/setuptools#4645)

distutils/distutils/ccompiler.py

Lines 983 to 992 in 3ac1adc

	def _make_relative(base):
	"""
	In order to ensure that a filename always honors the
	indicated output_dir, make sure it's relative.
	Ref python/cpython#37775.
	"""
	# Chop off the drive
	no_drive = os.path.splitdrive(base)[1]
	# If abs, chop off leading /
	return no_drive[os.path.isabs(no_drive) :]

In ccompiler.py, the function _make_relative aims to turn an arbitrary path into something "relative" in the sense that os.path.join can append it to some other path. In Python 3.13, os.path.isabs changed in a way that it now returns False for '/a/b' on Windows. This means that _make_relative('C:/a/b') only strips the drive but not the leading slash, and returns the bad '/a/b' (as opposed to 'a/b' with Python 3.12). Note that os.path.join still handles '/a/b' as an absolute path, os.path.join('build','/a/b') returns its second argument.

A suggestion (from someone not very familiar with the whole thing) would be to replace

return no_drive[os.path.isabs(no_drive) :]

with

return no_drive.lstrip(r'\/')