Set build_interface directories for integrated builds
Set build_interface directories for integrated builds

This makes it very easy to integrate entityx into a build.
I.e:
add_subdirectories(3rdparty/entityx)
target_link_libraries(my_target entityx[_shared])

This makes #include entityx/entityx.h just work.

jaynus/entityx,jaynus/entityx,jarrettchisholm/entityx,alecthomas/entityx,jaynus/entityx,jarrettchisholm/entityx,jarrettchisholm/entityx,alecthomas/entityx,alecthomas/entityx