patch 9.0.1544: recent glibc marks sigset() as a deprecated

Problem:    Recent glibc marks sigset() as a deprecated.
Solution:   Use sigaction() in mch_signal() if possible. (Ozaki Kiichi,
            closes #12373)
diff --git a/src/os_unixx.h b/src/os_unixx.h
index 3d1927b..94ec541 100644
--- a/src/os_unixx.h
+++ b/src/os_unixx.h
@@ -10,14 +10,7 @@
  * os_unixx.h -- include files that are only used in os_unix.c
  */
 
-/*
- * Stuff for signals
- */
-#if defined(HAVE_SIGSET) && !defined(signal)
-# define signal sigset
-#endif
-
-   // Sun's sys/ioctl.h redefines symbols from termio world
+// Sun's sys/ioctl.h redefines symbols from termio world
 #if defined(HAVE_SYS_IOCTL_H) && !defined(SUN_SYSTEM)
 # include <sys/ioctl.h>
 #endif