Enable demandimport only in scripts, not in importable modules (issue605)
This way other applications can choose if and when they want this feature,
because it might be problematic if those applications rely on ImportError.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
See http://www.selenic.com/mercurial/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.