doc: make i18n man and html
Allow overwrite LANGUAGE and LC_ALL make variables, for make i18n man and html.
After this patch, we can make i18n man and html by following command:
$ make clean all LANGUAGE=ja
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html