It's a hack because it's relying on the fact that without a shebang line or the appropriate magic number for your executable format, most platforms interpret sc

It's a hack because it's relying on the fact that without a shebang line or the appropriate magic number for your executable format, most platforms interpret scripts as a shell script; and it's using the fact that // can be treated as a comment delimiter in C or as a redundant way of referring to / in shell. So we interpret the first line as a shell script which calls out to make, to build the input file, then execute it, then exit so we don't try interpreting the rest of the C source as shell; and that first line of shell is interpreted as a comment in C.