define BOOST_PYTHON_NO_PY_SIGNATURES

wrapped C++ functions are hidden from the user anyway, and this define
reduces binary sizes a little bit.
optimize compiler flags, remove default libdirs

- remove -finline-functions, add -fvisibility-inlines-hidden.
- searching library paths from "$CC -print-search-dirs" should suffice,
  in case this fallback is still needed at all.