When installing, be prepared that template_dir may be relative.

When installing, be prepared that template_dir may be relative.

Since the Makefile in the template/ subdirectory is only used to install
the templates, we do not simply pass down the setting of template_dir
when it is relative, but construct the intended destination in a new
variable: A relative template_dir is relative to gitexecdir.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>