[issue22100] Use $HOSTPYTHON when determining candidate interpreter for $PYTHON_FOR_BUILD.

[Shiz]

Shiz added the comment:

After some further investigation it seems the which-clause fails when given a relative path. Since we should expect $HOSTPYTHON to be present in the first place, I modified the patch a bit to special-case the $HOSTPYTHON check, and fall back on the other interpreters as checked before.

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Added file: http://bugs.python.org/file36181/Python-hg-723e0a7c4914-fix-PYTHON_FOR_BUILD-detection-v2.patch

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