Use maintainer mode in configure

This will automatically rebuild Makefile and configure if needed,
preventing problems like the missing targets in
https://github.com/HoTT/HoTT/pull/286#issuecomment-30749614.  This means
that we now depend on having autobuild around to run ./configure and
make, though I think that systems without them will simply get warnings,
and not errors.
Remove support for ssreflect

The ssreflect support has been inactive for a long time now. It would
take some effort to make it work, and it is unlikely this will happen.
So I am ripping it out. If in the future anyone wants to put it back
in, they can have a look at the history and go from there.
Remove further traces of ssreflect.
Merge pull request #303 from ezyang/split-exercises

Split off HoTTBookExercises.v from HoTTBook.v.
Add HoTTBookExercises.v to Makefile_targets.mk

This was forgotten in #303.
Merge pull request #292 from andrejbauer/master

Remove ssreflect "support" which isn't working anyhow.
Merge pull request #290 from JasonGross/enable-maintainer-mode

Use maintainer mode in configure
Move developer note to non-rendered section.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>
Solutions for 1.2-3, expand 1.1 solution.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>