diff --git a/Makefile b/Makefile
index dd3d5203f9..c956ce9437 100644
--- a/Makefile
+++ b/Makefile
@@ -846,6 +846,7 @@ ifeq ($(uname_S),IRIX)
 	SNPRINTF_RETURNS_BOGUS = YesPlease
 	SHELL_PATH = /usr/gnu/bin/bash
 	NEEDS_LIBGEN = YesPlease
+	THREADED_DELTA_SEARCH = YesPlease
 endif
 ifeq ($(uname_S),IRIX64)
 	NO_SETENV=YesPlease
@@ -859,6 +860,7 @@ ifeq ($(uname_S),IRIX64)
 	SNPRINTF_RETURNS_BOGUS = YesPlease
 	SHELL_PATH=/usr/gnu/bin/bash
 	NEEDS_LIBGEN = YesPlease
+	THREADED_DELTA_SEARCH = YesPlease
 endif
 ifeq ($(uname_S),HP-UX)
 	NO_IPV6=YesPlease