libgpgme: handle argp-standalone

libgpgme has an internal implementation of argp parsing for when the
libc lacks such an implementation (i.e. uClibc).
However if it detects argp.h the internal implementation is disabled and
for a uClibc-based toolchain that breaks the build.
This happens when argp-standalone is installed since libgpgme never
tries to link to libargp.

Make it link against libargp, which also saves some target size. Fixes:
http://autobuild.buildroot.net/results/33c/33c5776d2b3f7fa276a27f7602eb71c4d77c7b62/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>