* libc/include/sys/signal.h [__CYGWIN__]: Issue error message

when _CYGWIN_TYPES_H is not defined.

newlib/libc/include/sys/signal.h

@@ -159,6 +159,11 @@ int _EXFUN(sigsuspend, (const sigset_t *));
 int _EXFUN(sigpause, (int));
 
 #if defined(_POSIX_THREADS)
+#ifdef __CYGWIN__
+#  ifndef _CYGWIN_TYPES_H
+#    error You need the winsup sources or a cygwin installation to compile the cygwin version of newlib.
+#  endif
+#endif
 int _EXFUN(pthread_kill, (pthread_t thread, int sig));
 #endif