when cross-compiling, the local fbc binary won't work.  use the system
bc instead as the demands of the .b script are simple enough.

https://bugs.gentoo.org/627126

--- a/bc/Makefile.in
+++ b/bc/Makefile.in
@@ -628,7 +628,7 @@ libmath.h: libmath.b $(fbcOBJ) $(LIBBC)
 	echo '{0}' > libmath.h
 	$(MAKE) global.o
 	$(LINK) -o fbc $(fbcOBJ) global.o $(LIBBC) $(LIBL) $(READLINELIB) $(LIBS)
-	./fbc -c $(srcdir)/libmath.b </dev/null >libmath.h
+	bc -c $(srcdir)/libmath.b </dev/null >libmath.h
 	$(srcdir)/fix-libmath_h
 	rm -f ./fbc ./global.o
 sbc.o: sbc.c