bug#16579: 24.3.50; Implement system_process_attributes on Darwin

[Steve Purcell]

I'm on OS X 10.11.4, and the patch applies cleanly for me but doesn't
compile:


  CC       sysdep.o
sysdep.c:3607:16: error: variable has incomplete type 'struct gcpro'
  struct gcpro gcpro1, gcpro2;
               ^
sysdep.c:3607:10: note: forward declaration of 'struct gcpro'
  struct gcpro gcpro1, gcpro2;
         ^
sysdep.c:3607:24: error: variable has incomplete type 'struct gcpro'
  struct gcpro gcpro1, gcpro2;
                       ^
sysdep.c:3607:10: note: forward declaration of 'struct gcpro'
  struct gcpro gcpro1, gcpro2;
         ^
sysdep.c:3618:3: warning: implicit declaration of function 'GCPRO2' is invalid 
in C99
      [-Wimplicit-function-declaration]
  GCPRO2 (attrs, decoded_comm);
  ^
sysdep.c:3705:3: error: use of undeclared identifier 'UNGCPRO'
  UNGCPRO;
  ^
1 warning and 3 errors generated.
make[2]: *** [sysdep.o] Error 1
make[1]: *** [src] Error 2
make: *** [bootstrap] Error 2