Be more selective in python version extraction in configure.
... where "selective" means | head -1. Some pythons spew not only their version but also the compiler version, so we were getting a multi-line value for PYTHON_VERSION in some places, which breaks the generated GNUMakefiles. configure must be setting some env var that is causing newer or custom-built pythons to produce this extra information. Anyway, head -1 should be a fine strategy.
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