Generate $module-public-headers.txt file, feed it to gtk-doc

Generate $module-public-headers.txt file, feed it to gtk-doc

Rather than having the gtk-doc build machinery have a list of header
files to exclude, change the GLib build to dump a list of public
header files generated from the maintained Makefile.am files for
each of glib/, gobject/, gio/.

Also, for glib, always install glib-unix.h, even on non-Unix
platforms, for the same reason we install gwin32.h even on Unix.

https://bugzilla.gnome.org/show_bug.cgi?id=651745