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/sysconfig.py b/Lib/distutils/sysconfig.py
index b51629e..01ee519 100644
--- a/Lib/distutils/sysconfig.py
+++ b/Lib/distutils/sysconfig.py
@@ -146,8 +146,15 @@
             prefix = plat_specific and EXEC_PREFIX or PREFIX
 
     if os.name == "posix":
-        libpython = os.path.join(prefix,
-                                 "lib", "python" + get_python_version())
+        if plat_specific or standard_lib:
+            # Platform-specific modules (any module from a non-pure-Python
+            # module distribution) or standard Python library modules.
+            libdir = sys.platlibdir
+        else:
+            # Pure Python
+            libdir = "lib"
+        libpython = os.path.join(prefix, libdir,
+                                 "python" + get_python_version())
         if standard_lib:
             return libpython
         else: