Use result of `which` when searching for python.
Use result of `which` when searching for python.

Fixes: https://github.com/nodejs/node-gyp/issues/667
Fixes: https://github.com/nodejs/node-gyp/issues/750
PR-URL: https://github.com/nodejs/node-gyp/pull/668
Reviewed-By: Ben Noordhuis <59bd0a3ff43b32849b319e645d4798d8a5d1e889@bnoordhuis.nl>

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