Rebuild source when allocator changes

To do so, the Makefile stores the contents of the MALLOC environment
variable in a file named .make-malloc. When the contents of this file
and the MALLOC variable are not equal, it forces a rebuild of the Redis
source tree.

A side-effect of this change is that choosing an allocator can now be
done using the single MALLOC variable instead of USE_TCMALLOC,
USE_JEMALLOC and so forth. These variables continue to work for
backwards compatibility.