This is one of the built-in variables for Make and can increase
portability on different operating systems (eg, on Windows, the
built-in $(RM) may be defined as "del" instead of "rm -f").
Move C code into subdirectory as it will soon be only of historical
interest. Add extra program to dump SBI instrument patches, switch
to using OPL2 instruments, and ignore "null" instruments (all-zero).