BLD: only compile fixed_interpolator.cpp once

[yut23]

PR Summary

Avoid a race condition on fixed_interpolator.o during parallel builds by building it only once and storing it in a static library.

Hopefully fixes #4278.

PR Checklist

• [ ] New features are documented, with docstrings and narrative docs
• [ ] Adds a test for any bugs fixed. Adds tests for new features.