skip the porting/utils.t unless in a git checkout

skip the porting/utils.t unless in a git checkout

Today I tried to build 5.16.0-RC0 on my Linode and I got this:

  ok 78 # skip utils/cpanp-run-perl executes code in a BEGIN block which fails for
   empty @ARGV
  not ok 79 - utils/cpanp compiles
  # Failed test 79 - utils/cpanp compiles at porting/utils.t line 81
  #      got "defined(%hash) is deprecated at /usr/local/lib/perl5/site_perl/5.10.
  0/Locale/Maketext/Lexicon.pm line 307.\n\t(Maybe you should just omit the define
  d()?)\nutils/cpanp syntax OK\n"
  # expected "utils/cpanp syntax OK\n"

Ugh.  We really don't want this to happen to somebody else, because this
test is "do not let the developer break stuff" not "make sure the install
works."