m4: Add compile / link check for qt
m4: Add compile / link check for qt

* m4/qt.m4 (FIND_QT): Check if a qt application can be compiled and
linked.

--
In case gpgme is cross compiled pkg-config may pick up qt
for the build system and not for the host system. To avoid that
we check that we can compile a qt program for host.

gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme,gpg/gpgme