[yocto] How to include libraries?

[Gary Thomas]

I have an image which needs to include the libcups2 package from
the cups recipe.  If I try adding this to my image recipe
   CORE_IMAGE_EXTRA_INSTALL_append = " libcups2"
I get this error:
   ERROR: Required build target 'my-demo-image' has no buildable providers.
   Missing or unbuildable dependency chain was: ['my-demo-image', 'libcups2']

However, I can get it to build with this
   MACHINE_EXTRA_RRECOMMENDS_append = " libcups2"

What's the difference between these two approaches and why doesn't the first
one work at all?

Thanks

-- 
------------------------------------------------------------
Gary Thomas                 |  Consulting for the
MLB Associates              |    Embedded world
------------------------------------------------------------