First doxygen support

Stolen from hwloc and updated/simplified.

Built by default when building in a developer's copy with needed tools.
Disabled by default when building from tarball.
make dist includes the PDF, html pages, and some manpages.

manpages have to be manually added to Makefile.am before they
are actually distributed. Only a couple ones are added already.
Note that most of them are just manpage-links to sections such
as netloc_api.3

HTML doesn't get installed for now (just like hwloc).

Only letter PDF is generated.