bpo-40854: Allow overriding sys.platlibdir via PYTHONPLATLIBDIR env-var (GH-20605) (GH-20725)

(cherry picked from commit 8f023a2f664f902a3d0b7a6f64d63afc0d1c15ae)

Co-authored-by: Sandro Mani <manisandro@gmail.com>
diff --git a/Include/cpython/initconfig.h b/Include/cpython/initconfig.h
index 69db2af..0a256d4 100644
--- a/Include/cpython/initconfig.h
+++ b/Include/cpython/initconfig.h
@@ -388,6 +388,7 @@
     wchar_t *base_prefix;       /* sys.base_prefix */
     wchar_t *exec_prefix;       /* sys.exec_prefix */
     wchar_t *base_exec_prefix;  /* sys.base_exec_prefix */
+    wchar_t *platlibdir;        /* sys.platlibdir */
 
     /* --- Parameter only used by Py_Main() ---------- */