Added handling of VCPP4LICENSE environment variable in
Dest::p4.pm. If that's present and pointing to a readable
file when a p4 daemon is started, a symlink will be created in
the (possibly newly created) p4root directory to point to the
p4 license file pointed to by VCPP4LICENSE. The 'make' target
test_all_p4_versions will cycle through each version of p4 and
p4d contained in the 'p4versions' directory, in both
unlicensed and (if VCPP4LICENSE present) licensed mode.