Fix more header-related warnings.

This fixes all current default warnings with GCC7 on Linux.  That does
not mean the code is anywhere close to clean.

rogue4/mach_dep.c

@@ -38,6 +38,11 @@
 #include <fcntl.h>
 #include <string.h>
 #include <errno.h>
+
+#ifdef CHECKTIME
+#include <unistd.h>
+#endif
+
 #include "rogue.h"
 
 int num_checks;		/* times we've gone over in checkout() */
@@ -266,9 +271,9 @@ checkout(int s)
 
 #ifdef CHECKTIME
 	checktime = (CHECKTIME * 60) / num_checks;
-#endif
 #ifdef SIGALRM
 	alarm(checktime);
+#endif
 #endif
 
 	chmsg(msgs[num_checks - 1], ((double) checktime / 60.0));