agent: Add possibility to NOT build kdm and/or gdm extensions

Since with GNOME 3.8 it's no longer possible to build extensions for GDM
we're now allowing to disable the build of the extensions for systems
not supporting it.
Additionally we're now allowing a better control via configure what to build.

Change-Id: I87745ea0e49860969d782c35e2e83cec12fff864
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>