How can I make recipes conditionally enabled?

[Carlos Rafael Giani <dv@pseudoterminal.org>]

Hello,

I have a question about a problem I've had several times in the past:

if I write a layer, and add .bbappends, then the layers with the 
corresponding .bb files become hard dependencies.
But lets say I want to write a BSP layer, and I add some machine 
specific Qt5 patches. Now everybody who wants to use
my layer must also include meta-qt5, even when they don't really want to 
use Qt5 anywhere. This is made even worse
if I have a dependency on meta-oe, which itself brings in a lot of 
modifications.

In short, I'd like to be able to make .bbappends and .bb files dependend 
on whether or not a layer is available. The hard
dependency is fine if it concerns things that are essential in my layer. 
But if its about optional things, it shouldn't cause
a build failure.

Thoughts?