Set of 'system headers' should be determined by system runtime version

[gavanderhoorn]

The set of system headers -- include-able Karel files that are provided as part of the FRVC packages -- is currently hard-coded in ktransw.py in is_system_header(..) (here).

In reality not all FRVC packages provide the same set of files, which could cause problems when a 'back-ported' header is used and which is then incorrectly considered to be a system header.

This should be fixed. Possible approaches are maintaining per-version lists in ktransw.py, or having ktransw check whether a header is really a system header by checking the version specific sub dir of WinOLPC (or wherever ktrans.exe and friends are located).

The latter is expensive, but as is_system_header(..) is only used when generating dependency files, the overhead is probably acceptable.