Fix warning on assigning from `const char*` to `char*`
Allow error reporting routines to accept varying number of arguments in modern style
Avoid passing `const char*` argument as `char*` in test-bison-nr
Fix warning about redefined macro when multiple scanners are used.
Fix a few "unused parameter" warnings
Remove unused variables in src/filter.c:filter_apply_chain
Remove unused argument and variable from src/scanopt.c:scanopt_err
Remove unused variable from src/filter.c:filter_fix_linedirs
Remove a few `unused variable` warnings
Fix `implicit function declaration` warnings in tests
Fix `unused function` warnings in tests
Fix two "signed/unsigned" warnings
Fix `label unused` warning
Fix paths to flex binary and include directory in tests