lib: introduce xpath_join

* lib/path-join.h, lib/path-join.c: New.
* lib/local.mk: Adjust.
* src/output.c: Use it.

lib/path-join.h

@@ -0,0 +1,38 @@
+/* Concatenate path components.
+   Copyright (C) 2018 Free Software Foundation, Inc.
+
+   This program is free software: you can redistribute it and/or modify it
+   under the terms of the GNU General Public License as published by the
+   Free Software Foundation; either version 3 of the License, or any
+   later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
+
+/* Written by Akim Demaille <akim@lrde.epita.fr>.  */
+
+#ifndef _PATH_JOIN_H
+# define _PATH_JOIN_H
+
+# ifdef __cplusplus
+extern "C" {
+# endif
+
+
+/* Concatenate two paths together.  PATH2 may be null, or empty, or
+   absolute: do what is right.  Return a freshly allocated
+   filename.  */
+char *
+xpath_join (const char *path1, const char *path2);
+
+
+# ifdef __cplusplus
+}
+# endif
+
+#endif /* _PATH_JOIN_H */