Change --build-w32 to old --build-w32-w64

* autogen.sh: Remove --build-w32-w64.

The build-w32-w64 command was never documented. Building
gpgex for 64 bit should be default for the convience
configure offered by autogen.sh

(cherry picked from commit 935576828cf0a77d952d8191def843e41512cbe0)