(svn r2882) Some fixes for MorphOS (by tokai)
- Don't track dependencies on system headers
- Add an include path
- Silence a warning regarding a bad signal prototype
- Remove executable flag from pictures
- Add proper OTTD icon

(svn r1236) MorphOS: added make release like in OSX (tokai)
MorphOS: cleaned up the code telling the difference between AmigaOS and MorphOS (tokai)