Makefile: make perf tests optional for profile build

The perf tests need a repository to operate on; if none is defined, we fall back to the repository containing our build directory. That fails, though, for an exported tarball of git.git, which has no repository. Since 5d7fd6d we run the perf tests as part of "make profile". Therefore "make profile" fails out of the box on released tarballs of v2.1.0. We can fix this by making the perf tests optional; if they are skipped, we still run the regular test suite, which should give a lot of profile data (and is what we used to do prior to 5d7fd6d anyway). Signed-off-by: Jeff King <peff@peff.net> Acked-by: Andi Kleen <ak@linux.intel.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>