PyPerf requires libpython.so (does not work with a "normal" python invocation).

It appears that `examples/cpp/pyperf/PyPerf` requires that a python program be "embedded" into a C program such that when it is running, it maps in libpython.so [1]. (Perhaps there is another way to have a Python program map in libpython?) Can it be made to work both with libpython and "just" python.

[1] Looking at the following line of code, `PyPerfUtil.cc:94`:
```
const static std::string kPy36LibName = "libpython3.6";
```

Start with examples/cpp/pyperf/PyPerf and PyPerfUtil.cc:94, then reproduce the issue using a normal Python invocation and an embedded interpreter. Trace how PyPerf detects or loads Python, and verify that the example works in both modes without requiring libpython.so.

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