As this developer has no access to MacOSX developer setups anymore,
Travis becomes the best bet to run performance tests on that OS.

However, on MacOSX /usr/bin/time is that good old BSD executable that
no Linux user cares about, as demonstrated by the perf-lib.sh's use
of GNU-ish extensions. And by the hard-coded path.

Let's just work around this issue by using gtime on MacOSX, the
Homebrew-provided GNU implementation onto which pretty much every
MacOSX power user falls back anyway.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Submitted-As: http://mid.gmane.org/ae429d2481111f7ad1927ef22e3a691d4c99ebd7.1466254995.git.johannes.schindelin@gmx.de