filenamecat: split into filenamecat-lgpl

filenamecat: split into filenamecat-lgpl

The concept of filenamecat is simple enough to use in an LGPL
manner, even though current clients are GPL for other reasons.
At any rate, it is nice to separate mfile_name_concat into its
own file so that the linker does not fail without xalloc_die.

* modules/filenamecat-lgpl: New module.
* modules/filenamecat (Files): Move library-safe files into
filenamecat-lgpl.
(Depends-on): Add filenamecat-lgpl.
(configure.ac): Declare witness.
* lib/filenamecat.h (file_name_concat): Only declare when using
GPL module.
* lib/filenamecat.c (longest_relative_suffix, mfile_name_concat):
Move...
* lib/filenamecat-lgpl.c: ...into new file.
* m4/filenamecat.m4 (gl_FILE_NAME_CONCAT_LGPL): New macro.
(gl_FILE_NAME_CONCAT): Use it.
* MODULES.html.sh (File system functions): Mention new module.

Signed-off-by: Eric Blake <ebb9@byu.net>