Building the clean target in src or libsrc should only delete stuff created by the make in those directories. Having both separated allows the Travis CI build to replace the native binaries with cross built binaries while keeping everything else.
- 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'.
