configure: check for argp

configure: check for argp

argp is only present in the GNU C library, so add a specific check for it in
configure. Also check if -largp is needed for linking against it.

Please run autoconf after applying.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Cc: George Dunlap <george.dunlap@eu.citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Cc: Olaf Hering <olaf@aepfle.de>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>
[ ijc -- ran autogen.sh ]