All include directives for gpsim headers are done using the gpsim/ prefix,
e.g. #include "gpsim/stimuli.h"
So the include directories passed to the compiler need to point to the
path before the gpsim subdir.
This has been broken since the first FindGPSim.cmake version, the existing
include directives only ever worked by chance if the installation prefix
of gpsim was the system one or one of the other dependencies whose include
dirs matched the one where the gpsim subdir was present.