classes/meta: Add DISTRO_FEATURES check for gtk+/gtk3+
If you currently do a DISTRO_FEATURES_remove = "x11" with OE-Core, you
see failures due to dependency problems. The work in resolving this was
partially completed a while back. This adds in the markup mainly for
gtk/gtk3+ recipes and means "bitbake world" will work successfully.
Rather than code the gtk/gtk+ specific distro features into each recipe,
a shared variable is used.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
