D5269: tests: sniff for /usr/local/bin/gmake and use it in test-fuzz-targets.t

[Yuya Nishihara]

> +  $ GMAKE=`which gmake`
> +  $ if [ -x $GMAKE ] ; then

Need to check whether `"$GMAKE"` is empty or not. And `which gmake` may
fail with 1.