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.
diff --git a/configure b/configure
index 1124412..139c2bf 100755
--- a/configure
+++ b/configure
@@ -632,6 +632,7 @@
 LIBPL
 PY_ENABLE_SHARED
 PLATLIBDIR
+BINLIBDEST
 LIBPYTHON
 EXT_SUFFIX
 ALT_SOABI
@@ -15334,7 +15335,11 @@
 fi
 
 
-# Check for --with-libdir-name
+
+BINLIBDEST='$(LIBDIR)/python$(VERSION)'
+
+
+# Check for --with-platlibdir
 # /usr/$LIDIRNAME/python$VERSION
 
 PLATLIBDIR="lib"
@@ -15353,6 +15358,7 @@
   { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
 $as_echo "yes" >&6; }
   PLATLIBDIR="$withval"
+  BINLIBDEST='${exec_prefix}/${PLATLIBDIR}/python$(VERSION)'
 else
   { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
 $as_echo "no" >&6; }