When SDL is built using CMake, Find_Package imports a target instead of
defining variables. If a target was imported we now define the include
path and linker flags variables from the target's properties.
Using imported targets is a best practice. Ideally, we should define an
imported target when we detect variables were defined.
However, the linker flags variable would need to be parsed into a
library path and a list of libraries, making that approach impractical.