Makefile: add helper for compiling with -fsanitize

Makefile: add helper for compiling with -fsanitize

You can already build and test with ASan by doing:

  make CFLAGS=-fsanitize=address test

but there are a few slight annoyances:

  1. It's a little long to type.

  2. It override your CFLAGS completely. You'd probably
     still want -O2, for instance.

  3. It's a good idea to also turn off "recovery", which
     lets the program keep running after a problem is
     detected (with the intention of finding as many bugs as
     possible in a given run). Since Git's test suite should
     generally run without triggering any problems, it's
     better to abort immediately and fail the test when we
     do find an issue.

With this patch, all of that happens automatically when you
run:

  make SANITIZE=address test

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>