Cmake: Extend find_package(Vala) macro and setup useful vars.

 * Setup with pkg-config information: - VALA_SHORTVER
                                      - VALA_LIBPKG
                                      - VALA_VAPIDIR
                                      - VALA_DATADIR
                                      - VALA_VAPIGEN
                                      - VALA_GEN_INTROSPECT
                                      - VALA_VALA_GEN_INTROSPECT
 * Use those variables to set Valama and Guanako constants to make build more
   generic.
 * Valama: Drop Config.VALA_DATADIR (if it is necessary use the value from
           Guanako directly).