bpo-40684: Fix make install for platlibdir=lib64 (GH-20736)


"make install" now uses the PLATLIBDIR variable for the destination
lib-dynload/ directory when ./configure --with-platlibdir is used.

Update --with-platlibdir comment in configure.
(cherry picked from commit 51ae31e5b93b986e57a7e18e25f981a6ffcdefb7)

Co-authored-by: Victor Stinner <vstinner@python.org>
4 files changed