Add support for building doxygen API documentation

Add support for building doxygen API documentation

Simple HTML docs generated from the doxygen comments.
After running 'make' or 'make doc', try firefox doc/html/index.html to
see it (if you have doxygen). It's also installed with 'make install'.
You can use --enable-docs or --disable-docs, or specifically
--with-doxygen or --without-doxygen (autodetected, default yes).

The docs are currently not distributed in the tarball, because I
couldn't make it work properly in all cases :/

Signed-off-by: Ran Benita <ran234@gmail.com>