Originally I used the usual variables (like $(CC) and $(CFLAGS) ) but after
all this doesn't make sense as any predefined values and/or user defined
settings can only be wrong.
- No complex shell logic.
- "Source file shadowing" for all targets via vpath.
- Dependency handling.
- True incremental build.
- Don't write into source directories.
- Easy cleanup by just removing 'wrk'.
