Makefile: enable THREADED_DELTA_SEARCH on IRIX and IRIX64

Since commit dcda361 removed the use of a variable length array from
builtin-pack-objects.c, it is now safe to compile with the threaded delta
search feature enabled.  Formerly, the MIPSpro 7.4.4m compiler warned that
variable length arrays should not be used with pthreads.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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