setting host_alias and build_alias from config.site doesn't work

[Dan Winship]

I put this in my config.site:

    case `pwd` in
        ...
        /home/danw/gnome/*-win32)
            build_alias=x86_64-unknown-linux-gnu
            host_alias=i686-w64-mingw32
            ;;
        ...

but it didn't work, because configure sets ac_tool_prefix and
cross_compiling before it reads in config.site.

I didn't investigate too deeply, but at first glance it looked like it
doesn't use those variables until after reading config.site, so moving
the code to set them after that shouldn't cause problems.

-- Dan