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/auto/configure b/src/auto/configure
index c273857..1998273 100755
--- a/src/auto/configure
+++ b/src/auto/configure
@@ -12658,7 +12658,7 @@
 	getpgid setpgid setsid sigaltstack sigstack sigset sigsetjmp sigaction \
 	sigprocmask sigvec strcasecmp strerror strftime stricmp strncasecmp \
 	strnicmp strpbrk strtol tgetent towlower towupper iswupper \
-	usleep utime utimes mblen ftruncate unsetenv
+	usleep utime utimes mblen ftruncate unsetenv posix_openpt
 do :
   as_ac_var=`$as_echo "ac_cv_func_$ac_func" | $as_tr_sh`
 ac_fn_c_check_func "$LINENO" "$ac_func" "$as_ac_var"