Python 3.13 fails to build under macOS, expat issue

[Benny Siegert <bsiegert%netbsd.org@localhost>]

Hi!

On a Mac running macOS 26.6.1, building lang/python313 fails with an error 
like "expat_config.h: no such file or directory".

On macOS, we use the built-in expat by default. However, the builtin 
version lacks an expat_config.h. I was able to fix the build by adding

PREFER.expat=pkgsrc

to mk.conf.

Should we make the textproc/expat builtin check also verify that 
expat_config.h is present? That would solve the problem once and for all.

--
Benny