Imported targets are highly preferred over the individual variables
for includes and libs because they will be used in a coherent way
and any spelling mistakes or unavailability won't go unnoticed.
Also it will prevent bugs like conan-io/conan-center-index#16900
or using mismatching header/libs combinations.
Signed-off-by: Gregor Jasny <gjasny@googlemail.com>
Reviewed-by: Andreas Schneider <asn@cryptomilk.org>