Enhance make clean
Enhance make clean

Make clean now is broken into two parts, build_clean and test_clean.
The build_clean target is responsible for cleaning up all generated
files from the marktab build process (invoked with make). The test_clean
target is similar but for all generated files from the test suite
(invoked with make test).

cknadler/marktab-c,cknadler/marktab-c,cknadler/marktab-c