[3.11] gh-98707: configure --with-system-libmpdec and --with-system-expat no longer include vendored headers (GH-98711) (#99391)

(cherry picked from commit 6abec1caffdba2e282b14fe57c6ce61974de4bbe)

Co-authored-by: Miro Hrončok <miro@hroncok.cz>