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/configure.ac b/src/configure.ac
index be869d1..2a3bc33 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -3746,7 +3746,7 @@
 	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)
 AC_FUNC_SELECT_ARGTYPES
 AC_FUNC_FSEEKO