[BUG Windows] make clean fails

[a2line]

I think this comes from a dune update (I'd say from the start of this year): make clean under Cygwin is removing files and subdirectories and lets the _build directory empty but fails to remove the dir itself. It shouldn't break the execution. The empty dir could be left as it is? (I can't do make clean distrib for eg.)

$ make clean
Cleaning…Error: rmdir(_build): Directory not empty
make: *** [Makefile:230: clean] Error 1