pythonGH-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 <[email protected]>

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)
 

configure

@@ -6249,7 +6249,9 @@ if test "$ac_sys_system" = "iOS"; then
 fi
 
 
+AC_SUBST([LIBDEST])
 AC_SUBST([BINLIBDEST])
+LIBDEST='$(LIBDIR)/python$(VERSION)$(ABI_THREAD)'
 BINLIBDEST='$(LIBDIR)/python$(VERSION)$(ABI_THREAD)'
 
 
@@ -6273,6 +6275,7 @@ if test -n "$withval" -a "$withval" != yes -a "$withval" != no
 then
   AC_MSG_RESULT([yes])
   PLATLIBDIR="$withval"
+  LIBDEST='${prefix}/${PLATLIBDIR}/python$(VERSION)$(ABI_THREAD)'
   BINLIBDEST='${exec_prefix}/${PLATLIBDIR}/python$(VERSION)$(ABI_THREAD)'
 else
   AC_MSG_RESULT([no])
@@ -6283,9 +6286,9 @@ fi],
 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])