Link correctly against libraries used

The mapistore backend uses libgnustep-base, libobjc, libNGObjWeb, but wasn't linked against to libraries. The SAML code has a few calls to gobject functions, so use pkg-config in configure to link to gobject-2.0. This makes it possible for dpkg-shlibdeps to calculate the correct library packages to depend on and will also make sure that ABI changes using symbol versioning will work correctly.

P.S. Pull request #10 that fixes a similar issue is also still open.