run-tests: move build/ directory to HGTMP
Before, running a test would give you a build/ directory in the root
of your Mercurial source tree. The directory had a full copy of the
the source, so a grep in '**/*.py' would find files inside build/.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://mercurial.selenic.com/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.