[3.13] gh-116622: Rename build variable MODULE_LDFLAGS back to LIBPYTHON (GH-122764) (GH-122842)

gh-116622: Rename build variable MODULE_LDFLAGS back to LIBPYTHON (GH-122764)

(LIBPYTHON was renamed MODULE_LDFLAGS in commit 7f5e3f04f.)
(cherry picked from commit 2f5c3b09e45798a18d60841d04a165fb062be666)

Co-authored-by: Malcolm Smith <smith@chaquo.com>