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/site.py b/Lib/site.py
index 2c71798..e981a14 100644
--- a/Lib/site.py
+++ b/Lib/site.py
@@ -334,13 +334,22 @@
             continue
         seen.add(prefix)
 
+        libdirs = [sys.platlibdir]
+        if sys.platlibdir != "lib":
+            libdirs.append("lib")
+
         if os.sep == '/':
-            sitepackages.append(os.path.join(prefix, "lib",
-                                        "python%d.%d" % sys.version_info[:2],
-                                        "site-packages"))
+            for libdir in libdirs:
+                path = os.path.join(prefix, libdir,
+                                    "python%d.%d" % sys.version_info[:2],
+                                    "site-packages")
+                sitepackages.append(path)
         else:
             sitepackages.append(prefix)
-            sitepackages.append(os.path.join(prefix, "lib", "site-packages"))
+
+            for libdir in libdirs:
+                path = os.path.join(prefix, libdir, "site-packages")
+                sitepackages.append(path)
     return sitepackages
 
 def addsitepackages(known_paths, prefixes=None):