Use Ubuntu's GCC to compile 1.9.3

Instead of using newer mingw-w64 compiler, we use GCC from Ubuntu that
matches as close as possible the characteristics of DevKit used for
Ruby 1.9.3 on Windows (GCC 4.5.2)

We do this to avoid cross-compilation of some gems detect
characteristics of mingw-w64 headers that will not be present on
Windows version of 1.9.3

This happen when using mingw-w64 to cross-compile projects like
EventMachine that uses some settings of mingw-w64 headers not present
in Windows counterpart for Ruby 1.9.3

Doing this we reduce the chances of messing things up when
cross-compiling gems.