gettext.mk: Fix installation rules with some shells.

The following syntax is not portable to certain shells:

  for f in ; do stuff; done

When no translations are installed, a command just like this is
generated in the installation rules.  Instead of doing nothing
as desired, HP-UX 11 /bin/sh, heirloom-sh, and presumably also
old Solaris /bin/sh will exit with an error.

To work around this problem, we can stuff the list into a shell
variable and then use that in the for loop.