Add NEWS entries and acknowledgement.

Author: matejcik

Doc/whatsnew/3.9.rst

@@ -131,6 +131,15 @@ Build and C API Changes
   way to call a callable Python object without any argument.
   (Contributed by Victor Stinner in :issue:`37194`.)
 
+* Introduce a ``platsubdir`` sysconfig variable that contains the name of the
+  platform-specific "lib" directory. This is used to construct paths that do
+  not differ by prefix, i.e., ``/usr/lib`` vs ``/usr/lib64``. The
+  ``posix_prefix`` install scheme is changed to take this into account.
+
+  By default, this is ``basename $libdir``, but can be overriden by
+  ``./configure --with-custom-platsubdir``.
+  (Contributed by matejcik in :issue:`1294959`.)
+
 
 
 Deprecated

Misc/ACKS

@@ -1044,6 +1044,7 @@ Sidney San Martín
 Westley Martínez
 Sébastien Martini
 Roger Masse
+Jan Matějek
 Nick Mathewson
 Simon Mathieu
 Laura Matson

Misc/NEWS.d/next/Build/2019-07-14-14-12-04.bpo-1294959.RGk38C.rst

@@ -0,0 +1,3 @@
+On 64bit Linux systems that use ``$prefix/lib64``, this is now the default
+install location for the standard library. Python will still be able to
+import modules installed in ``$prefix/lib``

Misc/NEWS.d/next/Library/2019-07-14-14-13-05.bpo-1294959.VosXOa.rst

@@ -0,0 +1,7 @@
+Introduce a ``platsubdir`` sysconfig variable that contains the name of the
+platform-specific "lib" directory. This is used to construct paths that do
+not differ by prefix, i.e., ``/usr/lib`` vs ``/usr/lib64``. The
+``posix_prefix`` install scheme is changed to take this into account.
+
+By default, this is ``basename $libdir``, but can be overriden by
+``./configure --with-custom-platsubdir``.