This brings Ribchester into line with Canterbury, GLib, libdbus and
similar: you are meant to include <ribchester/ribchester.h>,
regardless of whether that comes from $(top_srcdir), $(top_builddir)
or the installed $(includedir)/$(PACKAGE)-$(API_VERSION).

To retain backwards compatibility with current versions of Frome,
we add $(includedir)/$(PACKAGE)-$(API_VERSION)/ribchester to the
include path so that Frome can continue to #include <ribchester.h>
and have it work. This should be considered deprecated.

Signed-off-by: Simon McVittie <smcv@collabora.com>

Differential Revision: https://phabricator.apertis.org/D6657