bpo-40684: Fix make install for platlibdir=lib64

[vstinner]

"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.

https://bugs.python.org/issue40684

[vstinner]

Currently, --libdir=/usr/lib64 updates BINLIBDEST and DESTSHARED.

With this change, BINLIBDEST and DESTSHARED no longer depend on LIBDIR but always use $exec_prefix/$PLATLIBDIR/ path.

@hroncok @mcepl @ned-deily @zware: Would you mind to review this change? I'm not comfortable with changing the build system :-/

See https://bugs.python.org/issue40684 for the context and the bug.

[vstinner]

To reduce the risk of regression, one option would be to continue to use BINLIBDEST= $(LIBDIR)/python$(VERSION) by default, but use BINLIBDEST= ${exec_prefix}/${PLATLIBDIR}/python$(VERSION) with --with-platlibdir= option is used.

[vstinner]

To reduce the risk of regression, one option would be to continue to use BINLIBDEST= $(LIBDIR)/python$(VERSION) by default, but use BINLIBDEST= ${exec_prefix}/${PLATLIBDIR}/python$(VERSION) with --with-platlibdir= option is used.

Done. This PR has no more effect by default. Now it only changes BINLIBDEST when --with-platlibdir is used.

[mcepl]

+1 from me

[miss-islington]

Thanks @vstinner for the PR 🌮🎉.. I'm working now to backport this PR to: 3.9.
🐍🍒⛏🤖

[bedevere-bot]

GH-20761 is a backport of this pull request to the 3.9 branch.