Win32 build: Fix toolchain host for mingw
Win64 build: Add support for Fedora's Mingw 64bit toolchain. Add support for debug builds.

* use ARCH=64 env variable to build synfig for Win64
* use DEBUG=1 to build in debug mode

Example:
$ ARCH=64 DEBUG=1 bash ./autobuild/fedora-crosscompile-win.sh
Fix 'loses precission' error when building for Win64