g++ + mad fails t/run/mad.t

g++ + mad fails t/run/mad.t

For some reason, g++ causes $^X to have a relative path when called
with one, whereas gcc causes it to have an absolute path:

g++:

$ ./perl -le 'print $^X'
./perl

gcc:

$ ./perl -le 'print $^X'
/Users/sprout/Perl/perl.git-copy/perl

(This is on 32-bit darwin.)

This affects mad.t’s ability to find the current perl interpreter
after a chdir.