Here's an updated version in the form of a patch.

Features include:

- Ensure things are done in the right directory with pushd/popd

- Allow setting environment variables (for the life of the script) with:
     make test "PYTHON=..\PCbuild\python.exe"

- Emulate UNIX's `make -C <dir>` (cd dir; call make.bat %*, essentially)

- Choose x86 or x64 with "-64" command line switch

- Intelligently choose an interpreter for `make patchcheck` and `make test`

- Available targets are: build, clean, patchcheck, ready, test.  "ready" is implemented by Tools/buildbot/external[-amd64].bat; "make ready" sounds better and possibly more accurate than "make external" to me.

- Create a convenience "python.bat" script, which calls the newly built interpreter with all supplied arguments.  Created by :build, deleted by :clean (and added to .hgignore)


Everything seems to work ok for me, but my testing platforms are limited to a single Win7 laptop, currently.  This issue is also somewhat constrained by issue 17202; it is supposed to work without any change to .hgeol, but that can't be guaranteed.