A number of changes to configuration

Oops, I pushed further changes so this pull request has become a bit large. Here is a description of all the things it does:

1. *update `contrib/HoTTBook.v`* -- It looks like there is a chance that Marc Bezem's reading group might contribute to linking the HoTT book and the HoTT libraries, so I updated this and made sure it works.
2. Change the `hoq*` scripts so they fail gracefully in case `hoq-config` does not exist, or if `hoqide` is not set.
3. For Linux, add `lablgtk2` and `lablgtksourceview2` to the list of packages to be installed, so that `coqide` can be built.
4. Build `coqide` by default (by changing the target for `coq-HoTT` from `coqlight` to default)
5. Change the `./configure` script so that it does not look in `PATH` when `COQBIN` has been set.

I am not sure how Travis will react to these changes, nor do I know if the autogen stuff will realize by itself that `configure.ac` has changed. Do I need to do anything regarding that?

This pull request closes #258, closes #232, and closes #259.