Link against standalone argp library

Link against standalone argp library

argp is not part of POSIX standards and not provided by uClibc or
some other standard C libraries.  However, it is possible to link
against a breakout argp library.  One is provided at

    http://www.lysator.liu.se/~nisse/misc/

This patch tests if libc provides argp otherwise it adds the linker
flag -largp where needed in the build system.

Signed-off-by: Anthony G. Basile <blueness@gentoo.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>