Make NEURON builds reproducible

[JCGoran]

Overview of the feature

NEURON builds are currently not reproducible, i.e. deterministic. By this I mean if one runs the exact same build commands in the exact same source/build directories, the resulting generated files (plaintext and binary) are not identical. Optimistically, I would aim towards level 3 determinism as defined here (level 4 is probably not achievable since builds will always be machine-specific to some degree).

This would mostly be a QoL improvement to developers, since trying to find the difference in builds between 2 different commits currently requires post-processing the build files themselves (due to timestamps, hardcoded absolute paths, etc.).

Foreseeable Impact

Some refactoring needed.

• Area(s) of change: NMODL, nocmodl (default stays the same to maintain backwards compat), perhaps others?
• Possible issues: none that I can think of

Blockers: #3308