Some make implementations, e.g. on NetBSD 7.1, lack it; therefore
avoid it in order to prevent error messages during 'make clean'.
* Makefile.am (coverage-clean): Replace $(RM) by 'rm -f'.
* find/Makefile.am: Likewise.
* lib/Makefile.am: Likewise.
* locate/Makefile.am: Likewise.
* xargs/Makefile.am: Likewise.