setpgrp cmake check: use #if, not #if defined.

I still haven't got out of the habit of doing this the autotools way,
which doesn't work in cmake. cmake's HAVE_FOO variables are always
defined, and they take values 0 or 1, so testing them with 'defined'
will return the wrong value.

unix/pageant.c

@@ -330,9 +330,9 @@ void pageant_fork_and_print_env(bool retain_tty)
         /* Get out of our previous process group, to avoid being
          * blasted by passing signals. But keep our controlling tty,
          * so we can keep checking to see if we still have one. */
-#if defined HAVE_NULLARY_SETPGRP
+#if HAVE_NULLARY_SETPGRP
         setpgrp();
-#elif defined HAVE_BINARY_SETPGRP
+#elif HAVE_BINARY_SETPGRP
         setpgrp(0, 0);
 #endif
     } else {