[CMake] making a FooConfig.cmake with components

[Clinton Stimpson]

I'm writing a FooConfig.cmake to support

find_package(Foo COMPONENTS A B C)

I see that Foo_FOUND is always TRUE no matter what I try to set to from 
FooConfig.cmake.
How does a FooConfig.cmake tell the caller whether the components were 
all found?
I can make my own variables, but I wanted to know if there was an 
official way to do this.

Thanks,
Clint