Make sure external projects update

If the hash for an external project changes, issuing a make command at the superbuild level does not have any effect due to the `UPDATE_COMMAND ""`usage in our `ExternalProject/*.cmake` files.

It even will not work using `make -B <target>`. Removing the empty update commands revealed a bug in the handling of the `CMAKE_EXE_LINKER_FLAGS` variable which caused every CTK executable to be re-linked if the CTK-build inner project was re-configured.

These changes will trigger the configure/build/install/etc. steps for the external projects which results in a longer CTK superbuild configure run. But it reliably updates projects if their hash changed, so IMHO this is preferable.

@jcfr Can you please review?