make-package-cross-buildable libkarma fails to cross build from source, because it fails relinking the library during make install. This is because it uses the build architecture compiler there as dh_auto_install (unlike dh_auto_build) does not pass cross compilers along. This relinking is entirely unnecessary though and is due to depending on $(LIBDIR) which is always modified. Turning that dependency into an order-only dependency avoids the unnecessary relinking and thus makes libkarma cross buildable. It also builds slightly faster.
