run-tests: fix _checkhglib() so it's correct when using --with-hg.
Since this makes the warning less likely, upgrade it to a "real"
warning to stderr.
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.