build and install api docs with doxygen

[David Bremner]

In my testing this version works ok with and without doxygen, and in
and out of tree. But I only tested in Debian jessie (testing), so other tests would be welcome.

It's a bit lame that we have to generate doxygen config file snippet
on the fly just to support out of tree builds, but doxygen seems to
have very limited support for options on the command line.

The second two patches are essentially cosmetic, although without
patch 3/3 the docs are bit eye-burningly-ugly