build: allow build with system python 3
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
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
commit message follows commit guidelines
Affected core subsystem(s)
build