moarvm: use pkgconfig to find libraries and headers

MoarVM's Configure.pl script uses pkgconfig to find libraries and
headers, but it uses the one installed in the host machine which is
wrong and can lead to errors of unsafe include paths.

Instead, make it depend on the host-pkgconf package and specify the path
to our pkgconfig binary by using the --pkgconfig configure option.

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>