Enable building of the HTML manual by default unless not building from
Git and with an existing local copy (which is included in distribution
tarballs).
This makes sure we can install the HTML manual without having it
checked in VCS, yet not require rst2html for tarball builds.
* Add `--enable-html-docs` and `--enable-pdf-docs` arguments
(off by default)
* Add `RST2HTML`, `RST2LATEX` and `PDFLATEX` to override path to these
tools.
* Add HTML and PDF doc status messages to configure summary