[pypy-issue] [issue1723] test runner filling temp directory with _testcapi modules

[mattip]

mattip <matti.picus at gmail.com> added the comment:

What do you think of the hash idea to build once, use many times?

fwiw, we support various non-documented environmental variables to control things 
like where to put files (PYPY_USESSION_DIR), compiler (CC) and probably others, 
"someone" should document them in pypy/doc/config/index.rst

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