patch 8.1.2326: cannot parse a date/time string

Problem:    Cannot parse a date/time string.
Solution:   Add strptime(). (Stephen Wall, closes #)
diff --git a/src/config.h.in b/src/config.h.in
index f9ae0bb..e17f036 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -206,6 +206,7 @@
 #undef HAVE_STRNCASECMP
 #undef HAVE_STRNICMP
 #undef HAVE_STRPBRK
+#undef HAVE_STRPTIME
 #undef HAVE_STRTOL
 #undef HAVE_CANBERRA
 #undef HAVE_ST_BLKSIZE