makefile: add a build-rhg option
This gives an easy action to build the rhg-binary. This will be useful for the
`install-rhg` action in the next changeset.
Differential Revision: https://phab.mercurial-scm.org/D10193
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.