meson: wire up benchmarking options

meson: wire up benchmarking options

Wire up a couple of benchmarking options that we end up writing into our
"GIT-BUILD-OPTIONS" file. These options allow users to control how
exactly benchmarks are executed.

Note that neither `GIT_PERF_MAKE_COMMAND` nor `GIT_PERF_MAKE_OPTS` are
exposed as a build option. Those options are used by "t/perf/run", which
is not used by Meson.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>