Unfortunately your fix breaks other platforms, where 'long' works and is large enough to contain a fixnum. I will try to find another way of fixing this. Sorry for the noise.
I have found a couple of places where ECL uses mpz_set_si instead of _ecl_big_set_fixnum() and also others where mpz_mul_ui instead of higher level functions from big.d. I uploaded some fixes. Could you have a look and report back whether ECL now builds in Windows64?
If you have long logs or reports to share, we can turn this conversation into a private one outside the mailing list.