Remove redundant source files
- Remove dependence on ancient local copy of OSLib header files - requires a few tweaks to adjust for changes in OSLib types.
- Remove some unused files from the local copy of the Support library.
- Remove unused (and empty) file sources.h.defmod_tb.
Based on !1 (merged) in order to avoid introducing merge conflicts when rebasing back and forth from current master branch, so merge that first.