Prevent the preprocessor from replacing std{in,out} with macro

Trying to build on alpine (which uses musl instead of glibc) would break
as the preprocessor replaces stdin->(stdin) which fails to compile.

After discussion with @ankon he found:
https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20130506/173524.html

judge/runpipe.cc

@@ -641,22 +641,22 @@ struct state_t {
         tie(read_end, write_end) = make_pipe();
         logmsg(LOG_DEBUG, "setting up pipe #%ld (fd %d) -> proxy (fd %d)", i,
                write_end, read_end);
-        process.stdout = write_end;
+        process.pstdout = write_end;
         process.process_to_proxy = read_end;
         set_non_blocking(process.process_to_proxy);
 
         tie(read_end, write_end) = make_pipe();
         logmsg(LOG_DEBUG, "setting up pipe proxy (fd %d) -> #%ld (fd %d)",
                write_end, j, read_end);
         other.proxy_to_process = write_end;
-        other.stdin = read_end;
+        other.pstdin = read_end;
       } else {
         // No proxy: direct communication.
         tie(read_end, write_end) = make_pipe();
         logmsg(LOG_DEBUG, "setting up pipe #%ld (fd %d) -> #%ld (fd %d)", i,
                write_end, j, read_end);
-        process.stdout = write_end;
-        other.stdin = read_end;
+        process.pstdout = write_end;
+        other.pstdin = read_end;
       }
     }
   }