contrib: set HAVE_WIN64 in the Makefile

It's tied to the ARCH value which comes from the HOST which is hardcoded in the Makefile.

We may hardcode even more values based on BUILD which is also hardcoded in the Makefile. If it's not set the boostrap fails.

Having less conditional parts in the Makefile might make it faster. Right now it's very slow testing a lot of things in the shell.

The "bootstrap" is actually more a configure script for the contribs. It needs to be called again for each build environment/target changes.