bpo-43667: Fix broken Unicode encoding in non-UTF locales on Solaris (GH-25096)

diff --git a/pyconfig.h.in b/pyconfig.h.in
index 6e54d55..63438d8 100644
--- a/pyconfig.h.in
+++ b/pyconfig.h.in
@@ -748,6 +748,10 @@
 /* Define to 1 if you have the `nice' function. */
 #undef HAVE_NICE
 
+/* Define if the internal form of wchar_t in non-Unicode locales is not
+   Unicode. */
+#undef HAVE_NON_UNICODE_WCHAR_T_REPRESENTATION
+
 /* Define to 1 if you have the `openat' function. */
 #undef HAVE_OPENAT