expat: Activate POSIX APIs even without compiler extensions
Compile with the preprocessor definitions necessary for the arc4random
family so
it's available even when compiler extensions are not enabled.
Similar things are done in cmbzip2, cmcurl, cmlibarchive, cmliblizma and
cmlibuv.
This issue surfaced on a nightly bot after glibc 2.36 added arc4random
functions. cmlibarchive defines the necessary macro, but also relies on
HAVE_ARC4RANDOM_BUF
. cmlibarchive's check with the necessary macro defined was
skipped due to cmexpat running the same check before, but without the macros,
and it being cached.
This extends !7320 (merged).
Issue: #20454
Backport: release
Topic-rename: expat-c-no-extensions
Edited by Brad King