Check and define AS (for cross/biarch compiler support).

diff --git a/Make.rules.in b/Make.rules.in
index 4679c1b..2b6181e 100644
--- a/Make.rules.in
+++ b/Make.rules.in
@@ -48,6 +48,7 @@
 TOOLSDIR  = @TOOLSDIR@
 DIVINCL   = -I$(SRCDIR) -I. -I$(TOPSRCDIR)/include -I$(TOPOBJDIR)/include $(EXTRAINCL)
 ALLCFLAGS = $(DIVINCL) $(CFLAGS) $(DEFS) $(OPTIONS)
+AS        = @AS@
 LD        = @LD@
 LDFLAGS   = @LDFLAGS@
 LDCOMBINE = $(LD) -r