Patch for musl support

[Barak A. Pearlmutter]

Happy to in principle, but might it not be better to put in util.cpp
right after #include <ftw.h> or something like that:

#ifndef FTW_STOP
#define FTW_STOP 1
#endif
etc for:
...
#define FTW_CONTINUE 0
...
#define FTW_ACTIONRETVAL 16