livepatch-tools: add -largp option when required

livepatch-tools: add -largp option when required

crate-diff-object makes use of argp library, and depending on the libc
used by the system (ie: musl or BSD libc) argp is a separate library
and requires the addition of -largp to the build rune.

Introduce some shell logic to detect whether -largp is required for
linking create-diff-object.

I haven't done this as a reusable macro because I'm not sure there's
much point in doing so, the only library we need to test for is argp,
anything else is likely to be a mandatory library flag that doesn't
require such testing (like libelf for example).

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>