bpo-1294959: Add sys.platlibdir attribute (GH-18381)

Add --with-platlibdir option to the configure script: name of the
platform-specific library directory, stored in the new sys.platlitdir
attribute. It is used to build the path of platform-specific dynamic
libraries and the path of the standard library.

It is equal to "lib" on most platforms. On Fedora and SuSE, it is
equal to "lib64" on 64-bit systems.

Co-Authored-By: Jan Matějek <jmatejek@suse.com>
Co-Authored-By: Matěj Cepl <mcepl@cepl.eu>
Co-Authored-By: Charalampos Stratakis <cstratak@redhat.com>
diff --git a/Lib/distutils/command/install.py b/Lib/distutils/command/install.py
index c625c95..aaa300e 100644
--- a/Lib/distutils/command/install.py
+++ b/Lib/distutils/command/install.py
@@ -30,14 +30,14 @@
 INSTALL_SCHEMES = {
     'unix_prefix': {
         'purelib': '$base/lib/python$py_version_short/site-packages',
-        'platlib': '$platbase/lib/python$py_version_short/site-packages',
+        'platlib': '$platbase/$platlibdir/python$py_version_short/site-packages',
         'headers': '$base/include/python$py_version_short$abiflags/$dist_name',
         'scripts': '$base/bin',
         'data'   : '$base',
         },
     'unix_home': {
         'purelib': '$base/lib/python',
-        'platlib': '$base/lib/python',
+        'platlib': '$base/$platlibdir/python',
         'headers': '$base/include/python/$dist_name',
         'scripts': '$base/bin',
         'data'   : '$base',
@@ -298,6 +298,7 @@
                             'sys_exec_prefix': exec_prefix,
                             'exec_prefix': exec_prefix,
                             'abiflags': abiflags,
+                            'platlibdir': sys.platlibdir,
                            }
 
         if HAS_USER_SITE: