[feature] Find ways to pass CMake variables around

[runlevel5]

The current way of using add_subdirectory means that variables are scoped within that CMakeLists.txt scope. That makes life so hard to pass variables to different CMakeLists.txt. Why do care about passing variables around? It is because it would make life way easier for my work in #351 in which I could inject include_dir, libraries, etc from libraries and get it consumed across server/client and other components just like how ioq3 does upstream. If the team does not object to this proposal, I would like to have a go at it

[mgerhardy]

I think the XXX_LIBRARIES et all should be cache vars anway, no? also imo it would be cleaner to use interface libraries that you just link against without caring whether they are from internal sources or from Find_XXX stuff.

[mgerhardy]

add_library(somename::somename INTERFACE)
target_include_directories(somename::somename INTERFACE ${XXX_INCLUDE_DIRS})
target_link_libraries(somename::somename INTERFACE ${XXX_LINK_LIBRARIES})

like this and then just link against somename::somename

[runlevel5]

In theory XXX_LIBRARIES could be cached, or we could use set_property(GLOBAL) but I guess the first is a bit cleaner. Okay let me give that approach a try first