Update P4Perl to build against 2007.2 API.
The 2007.2 API has
a more traditional directory structure and P4Perl needs to
take that into account. This change also makes Makefile.PL
check that the version of the API being used is less than
or equal to the highest API version known when this version
was written.
Hopefully this will prevent people from struggling to build
older (stable) versions of P4Perl against recent APIs only
supported by the development builds.