-Make up for the missed DLL_EXPORT-it's actually needed for all GLib DLL
builds, omitting this caused problems to surface due to recent works to
make GDBus work on Windows.
-Also use the FFI_BULIDING macro for GObject builds as the suggessted
workaround for using static LibFFI builds (as we do now)-please see
ffi.h.in. This will fix the build of GObject against LibFFI 3.0.11, but
it is probable that this will change at some point for LibFFI.