makefile: stop setting unused HGEXTDIR variable during osx build
The variable was added in
a38ed42cd23c (osx: include chg by default,
2017-03-20), but I can't find any others references to the variable in
that commit or in any other commits.
Differential Revision: https://phab.mercurial-scm.org/D8915
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.