Cygwin build failures

[me-and]

Currently, Cygwin builds are failing; it looks like the relevant #defines in Cygwin have changed at some point. Specifically, when trying to compile, I get the following error:

  CC       src/main.o
In file included from /usr/include/sys/_pthreadtypes.h:12,
                 from /usr/include/sys/types.h:223,
                 from /usr/include/stdio.h:61,
                 from src/main.c:4:
src/main.c: In function ‘main’:
src/main.c:162:27: error: ‘cpu_set’ undeclared (first use in this function); did you mean ‘cpu_set_t’?
  162 |                 CPU_ZERO(&cpu_set);
      |                           ^~~~~~~
src/main.c:162:27: note: each undeclared identifier is reported only once for each function it appears in
make: *** [Makefile:507: src/main.o] Error 1

I have a fix, which I'm going to put into a PR now…