build: allow build with system python 3

[forivall]

Backport from nodejs/node#16058

When the system python is python 3, configure now creates a directory
with a symlink called 'python' to python2, uses it when it calls
run_gyp, and puts it in config.mk so that it propagates to everything
that make launches

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

build

[tniessen]

Wouldn't it be a good idea to just cherry-pick it from upstream once it has landed there? Or include it in a normal upstream-to-downstream update? (I don't really know the policy of those updates, it just makes sense in my head.)

[TimothyGu]

Given the number of approvals/:+1:s there, it seems to me that the patch will land in Node.js pretty soon after the ordinary 48/72-hour waiting period. When that is done, we can then include it in an ordinary pull-from-upstream.

[forivall]

Yup, it's been merged there. 🎉