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.ac b/configure.ac
index 84d1f00..30856c8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -4770,7 +4770,11 @@
 fi
 
 
-# Check for --with-libdir-name
+AC_SUBST(BINLIBDEST)
+BINLIBDEST='$(LIBDIR)/python$(VERSION)'
+
+
+# Check for --with-platlibdir
 # /usr/$LIDIRNAME/python$VERSION
 AC_SUBST(PLATLIBDIR)
 PLATLIBDIR="lib"
@@ -4787,6 +4791,7 @@
 then
   AC_MSG_RESULT(yes)
   PLATLIBDIR="$withval"
+  BINLIBDEST='${exec_prefix}/${PLATLIBDIR}/python$(VERSION)'
 else
   AC_MSG_RESULT(no)
 fi],