[cmake-developers] Depends information in buildsystem files

[Stephen Kelly]

Hi there,

I've pushed a branch with some addtional fixes for various issues in 
generator expressions. 

One of the issues relates to depends information in the build dir. I fixed 
the situation in the makefile generator by expanding the COMPILE_DEFINITIONS 
before writing out the DependsInfo.cmake file. The Ninja generator doesn't 
use such a file, but from my porcelain test it seems to already detect the 
INTERFACE content appropriately. I didn't check how that works.

It occurs to me though that this could be not just one bug but a class of 
bugs. A similar issue could exist for the other generators, and anywhere 
else that cmake generates 'buildsystem artifacts' which make need to be 
evaluated as generator expressions, or may need to take INTERFACE content of 
dependencies into account. 

I wouldn't know what to look for in other generators. Any idea if a similar 
issue could occur elsewhere?

Another possible candidate is <target>_LIB_DEPENDS in the cache. Currently, 
cmTarget::AddLinkLibrary aborts if a generator expression is found before 
populating that cache variable. Is that appropriate?

Thanks,

Steve.