enhance Makefile for language translators
To update a po-file just do 'make i18n/xx.po'. No other po-file will
be touched.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://www.selenic.com/mercurial/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.