tests: sniff for /usr/local/bin/gmake and use it in test-fuzz-targets.t
This isn't as robust as it probably should be, but for now it'll get
the job done on the buildbots.
Differential Revision: https://phab.mercurial-scm.org/D5269
#require docutils gettext $ $TESTDIR/check-gendoc it checking for parse errors