Improve Docutils (and friends) support in Autotools

* 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
Use improved Docutils Autotools support

FIXME: The `install-html-local` rule might be wrong since it's
copying `$(srcdir)/geany.html` instead of `$(builddir)/geany.html`
because for some reason generated `geany.html` is checked-in to
version control in the source `doc` directory.
Add `--enable-html-docs` to distcheck flags

FIXME: `--enable-pdf-docs` not added because I can't make it pass
distcheck due to quirks of `pdflatex` (see comments in
`doc/Makefile.am`)