Fix build where GObject & deps libraries are not in default library path

pkg_check_modules sets GOBJECT_LIBRARIES "only the libraries (w/o the '-l')"
as read from the pc files. So without the -L informations, resulting
in the libraries not found if in non-default paths.

Just passing GOBJECT_LDFLAGS as already done also for GCONF_LDFLAGS is
a workaround.