GTK+: Fix dist and build on Windows

Hi,

In gtk/Makefile.am, there are some private headers that are missed from the dist tarball, which are needed for the build. In addition, there are some assumptions in the code that have hard dependencies on libraries and compiler features that are optional, at least for Windows.

This MR attempts to remedy these issues, so that things will build and run on Windows.

With blessings, thank you!