CMake/FindMPI: Drop unnecessary and incorrect use of GetPrerequisites
CMake/FindMPI: Drop unnecessary and incorrect use of GetPrerequisites

The call to the GetPrerequisites is_file_executable function passes the
name of a CMake variable instead of its value.  Therefore the function
has always failed and caused the search for the compiler name to run
even with an absolute path.  Switch to using if(IS_ABSOLUTE) instead and
drop use of GetPrerequisites.

keithroe/vtkoptix,SimVascular/VTK,sumedhasingla/VTK,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,sumedhasingla/VTK,sumedhasingla/VTK,sumedhasingla/VTK,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,sumedhasingla/VTK,keithroe/vtkoptix,sumedhasingla/VTK,SimVascular/VTK,sumedhasingla/VTK,sumedhasingla/VTK