Use python binary instead of libpython when it's a PIE

Presumably, this would let us share precompilation cache between PyCall and PyJulia in more situations [1]: https://github.com/conda-forge/python-feedstock/issues/222#issuecomment-436507431

[1] Not all situation. For example, non-PIE statically linked `python` still won't work.

Start by tracing how PyCall selects and uses libpython versus the python binary, then read the linked conda-forge discussion about PIE builds. Done means PIE Python builds use the binary in cases where this enables shared precompilation caches, while non-PIE statically linked Python remains unsupported.

Vom Indexierungsmodell aus dem Issue-Text verfasst.