OK, there seems to be a bit too much make magic for building
bitstring_c.o. I confirmed that the 'linux64' string wasn't in the
CFLAGS. So it seems there are some strange implicit make rules being
invoked to build bitstring_c.o.
I added an explicit rule to the Makefile:
bitstring_c.o: bitstring_c.c
$(CC) $(CFLAGS) -c $^
And it builds fine. I'd suggest adding that to the Makefile.in.
BTW: I'm on Linux Mint (which is basically Ubuntu 11.04) 64 bit.
Phil