Fix some reported compilation problems by including string.h with _GNU_SOURCE defined rather than prototyping strsignal ourselves. b=177912 sr=bryner

xpfe/bootstrap/nsSigHandlers.cpp

@@ -67,8 +67,11 @@
 #include "nsIAppShellService.h"
 #include "nsAppShellCIDs.h"
 static NS_DEFINE_CID(kAppShellServiceCID,   NS_APPSHELL_SERVICE_CID);
-#else
-extern "C" char * strsignal(int);
+#elif defined(LINUX)
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE 1
+#endif
+#include <string.h>
 #endif
 
 #ifdef MOZ_WIDGET_PHOTON