qt5: fix clash with installed qt.

Apparently, passing HOMEBREW_PREFIX as -I and -L makes the Qt5
configure script pick up Qt4 headers. Hotfix it until I find
a solution that allows both installing to prefixes different than
/usr/local and to prefixes that already contain Qt4.

Closes #21211.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
python: Using setuptools 0.8 now
qt5: head branch is no longer 'master'
mercurial: 2.6.3 & use depends_on :python

* The `hg` binary is installed in `$(brew --prefix)/bin/hg`.
* Optional docs (hg help will still work) so that docutils is not
  needed.
* The `depends_on :python` is needed for software that needs
  the `Python.h`.

Fix #20929