GH-108819: fix LIBDEST not honoring --with-platlibdir (#133163)

* GH-108819: fix LIBDEST not honoring --with-platlibdir

We look for the pure-Python part of the standard library in
PLATSTDLIBDIR, which may not match the default LIBDIR subdir.

From ``getpath.py``:

```python
    ...
    STDLIB_SUBDIR = f'{platlibdir}/python{VERSION_MAJOR}.{VERSION_MINOR}{ABI_THREAD}'
    STDLIB_LANDMARKS = [f'{STDLIB_SUBDIR}/os.py', f'{STDLIB_SUBDIR}/os.pyc']
    PLATSTDLIB_LANDMARK = f'{platlibdir}/python{VERSION_MAJOR}.{VERSION_MINOR}{ABI_THREAD}/lib-dynload'
    ...
```

Signed-off-by: Filipe Laíns <lains@riseup.net>

* Add news

Signed-off-by: Filipe Laíns <lains@riseup.net>

* Always set LIBDEST and BINLIBDEST based on PLATLIBDIR

Signed-off-by: Filipe Laíns <lains@riseup.net>

* Add XXX comment on PLATLIBDIR default value

Signed-off-by: Filipe Laíns <lains@riseup.net>

* Regen configure

Signed-off-by: Filipe Laíns <lains@riseup.net>

---------

Signed-off-by: Filipe Laíns <lains@riseup.net>

Author: FFY00

Makefile.pre.in

@@ -166,7 +166,7 @@ WHEEL_PKG_DIR=	@WHEEL_PKG_DIR@
 
 # Detailed destination directories
 BINLIBDEST=	@BINLIBDEST@
-LIBDEST=	$(SCRIPTDIR)/python$(VERSION)$(ABI_THREAD)
+LIBDEST=	@LIBDEST@
 INCLUDEPY=	$(INCLUDEDIR)/python$(LDVERSION)
 CONFINCLUDEPY=	$(CONFINCLUDEDIR)/python$(LDVERSION)
 

Misc/NEWS.d/next/Build/2025-04-29-18-25-34.gh-issue-108819.qMUTRB.rst

@@ -0,0 +1,3 @@
+Honor :option:`--with-platlibdir` in the pure-Python standard library
+installation path, if ``PLATLIBDIR`` doesn't match the value used in
+``LIBDIR``.

configure

@@ -6420,15 +6420,10 @@ if test "$ac_sys_system" = "iOS"; then
   MODULE_DEPS_SHARED="$MODULE_DEPS_SHARED \$(PYTHONFRAMEWORKDIR)/\$(PYTHONFRAMEWORK)"
 fi
 
-
-AC_SUBST([BINLIBDEST])
-BINLIBDEST='$(LIBDIR)/python$(VERSION)$(ABI_THREAD)'
-
-
 # Check for --with-platlibdir
 # /usr/$PLATLIBDIR/python$(VERSION)$(ABI_THREAD)
 AC_SUBST([PLATLIBDIR])
-PLATLIBDIR="lib"
+PLATLIBDIR="lib"  # XXX: We should probably calculate the defauly from libdir, if defined.
 AC_MSG_CHECKING([for --with-platlibdir])
 AC_ARG_WITH(
   [platlibdir],
@@ -6445,19 +6440,22 @@ if test -n "$withval" -a "$withval" != yes -a "$withval" != no
 then
   AC_MSG_RESULT([yes])
   PLATLIBDIR="$withval"
-  BINLIBDEST='${exec_prefix}/${PLATLIBDIR}/python$(VERSION)$(ABI_THREAD)'
 else
   AC_MSG_RESULT([no])
 fi],
 [AC_MSG_RESULT([no])])
 
+AC_SUBST([LIBDEST])
+AC_SUBST([BINLIBDEST])
+LIBDEST='${prefix}/${PLATLIBDIR}/python$(VERSION)$(ABI_THREAD)'
+BINLIBDEST='${exec_prefix}/${PLATLIBDIR}/python$(VERSION)$(ABI_THREAD)'
 
 dnl define LIBPL after ABIFLAGS and LDVERSION is defined.
 AC_SUBST([PY_ENABLE_SHARED])
 if test x$PLATFORM_TRIPLET = x; then
-  LIBPL='$(prefix)'"/${PLATLIBDIR}/python${VERSION}${ABI_THREAD}/config-${LDVERSION}"
+  LIBPL='$(LIBDEST)'"/config-${LDVERSION}"
 else
-  LIBPL='$(prefix)'"/${PLATLIBDIR}/python${VERSION}${ABI_THREAD}/config-${LDVERSION}-${PLATFORM_TRIPLET}"
+  LIBPL='$(LIBDEST)'"/config-${LDVERSION}-${PLATFORM_TRIPLET}"
 fi
 AC_SUBST([LIBPL])