Yet another P4::SetProg fix.
Turns out the last change fixed
everything except the build script and it all worked...
unless you used the 2005.1 API which has a different format
for the Version file.
P4::SetProg is a no-op when P4Perl is built with an older API
so not identifying the API build properly breaks it thoroughly.
This change adapts P4Perl's setup script to support both
pre-2005.1 and 2005.1 formats, and gives the user the
chance to enter the API version manually if it can't be
automatically determined.