Use perl instead of sed so we can use whitespace matching

diff --git a/ash/Makefile b/ash/Makefile
index 7a1d3cd..4f7d879 100644
--- a/ash/Makefile
+++ b/ash/Makefile
@@ -143,7 +143,7 @@
 	mv lex.yy.c $@
 
 arith.h: arith.c
-	sed -n '/^#define ARITH/p' $< > $@
+	$(PERL) -ne 'print if ( /^\#\s*define\s+ARITH/ );' < $< > $@
 
 clean:
 	rm -f core $(CLEANFILES) $(OBJS) .*.d *.g