Flag to disable build of documentation.

Building the docs requires TeX, which brew doesn't install. Trying to build without it results in the following error:

```
configure: error: You have requested a build
of the documentation.  For this to work, you must have a functional install of
TeX and LaTeX.  kpsewhich is part of TeX.
Alternatively, you can disable building the documentation with
--disable-doc.
```

This patch still builds docs by default, it just makes it easier not to.