patch 8.1.1225: cannot create a pty to use with :terminal on FreeBSD

Problem:    Cannot create a pty to use with :terminal on FreeBSD.
Solution:   Add support for posix_openpt(). (Ozaki Kiichi, closes #4306,
            closes #4289)
diff --git a/src/config.h.in b/src/config.h.in
index 7fe65de..3f122fd 100644
--- a/src/config.h.in
+++ b/src/config.h.in
@@ -180,6 +180,7 @@
 #undef HAVE_NANOSLEEP
 #undef HAVE_NL_LANGINFO_CODESET
 #undef HAVE_OPENDIR
+#undef HAVE_POSIX_OPENPT
 #undef HAVE_PUTENV
 #undef HAVE_QSORT
 #undef HAVE_READLINK