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/gui_haiku.cc b/src/gui_haiku.cc
index 15c4389..c8b05e1 100644
--- a/src/gui_haiku.cc
+++ b/src/gui_haiku.cc
@@ -785,8 +785,8 @@
      * Apparently signals are inherited by the created thread -
      * disable the most annoying ones.
      */
-    signal(SIGINT, SIG_IGN);
-    signal(SIGQUIT, SIG_IGN);
+    mch_signal(SIGINT, SIG_IGN);
+    mch_signal(SIGQUIT, SIG_IGN);
 }
 
     void
@@ -1067,8 +1067,8 @@
      * Apparently signals are inherited by the created thread -
      * disable the most annoying ones.
      */
-    signal(SIGINT, SIG_IGN);
-    signal(SIGQUIT, SIG_IGN);
+    mch_signal(SIGINT, SIG_IGN);
+    mch_signal(SIGQUIT, SIG_IGN);
 
     if (menuBar && textArea) {
 	/*