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/PC/pyconfig.h b/PC/pyconfig.h
index 0aa4f95..02216b5 100644
--- a/PC/pyconfig.h
+++ b/PC/pyconfig.h
@@ -683,4 +683,6 @@
 /* Define if libssl has X509_VERIFY_PARAM_set1_host and related function */
 #define HAVE_X509_VERIFY_PARAM_SET1_HOST 1
 
+#define PLATLIBDIR "lib"
+
 #endif /* !Py_CONFIG_H */