avoid to compare to 1 when a simple if on the variable is enough --> was...

avoid to compare to 1 when a simple if on the variable is enough --> was causing strange behaviors in FindSTARPU.cmake for example (without MPI, STARPU_starpu_mpi.h_INCLUDE_DIRS-NOTFOUND was appending in the list of STARPU_INCLUDE_DIRS even if mpi was not required)