Update P4Ruby to support the new SetProg() method in the 2004.2 API.
Whilst the new 'P4#prog=' method is always available, it's only
functional if P4Ruby is built with a 2004.2 or later API.
The build system got a bit of tidying up to support this change and
the API version is now detected automatically if possible. I've also
removed the --apilibdir and --apiincludedir flags as they complicate
matters and I never use them and I don't believe anyone else does
either.
There are also some minor doc formatting tweaks to go along with
the added documentation for prog=.