fix Makefile.PL when no global .gitconfig exists

If no global .gitconfig file exists, git config --global will die with
an error.  As long as we don't specify the global flag, a nonexistent
global config will be ignored, but it will still fail if a broken $HOME
is set.
Merge pull request #53 from haarg/fix-no-global-config

fix Makefile.PL when no global .gitconfig exists