From Martin Storsjö:

* configure.ac (W64_ABI): New variable, set when compiling for
64-bit M$ windows.
* config.m4.in (W64_ABI): Define, from configure substitution.

Rev: nettle/ChangeLog:1.240
Rev: nettle/config.m4.in:1.3
Rev: nettle/configure.ac:1.52