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.
In the CMake generator. So the linker can resolve the symbols it marked
as unresolved as it walks the libraries.
Fixes building ResidualVM with msys2 from Qt Creator on Windows.
