This adds a header to the girepository library, which is then included
either directly or indirectly by the other headers so that all the public
symbols (and the 2 symbols in gitypelib-internal.h used by the tools) are
decorated by a macro, that can later be used to export the symbols and also
to be used to display compile-time warnings for usage of deprecated APIs,
which is like what is now being done in GLib (and GTK+, Clutter, and so
on).

This marks the first step that we begin to stop depending on the .symbols/
.def files to export the symbols.

https://bugzilla.gnome.org/show_bug.cgi?id=732669