Disable Fortran on MacOS systems

Build failed on OSX 10.6.8 (snow leopard) because fortran compatible compiler not available for Trilinos.

From http://trilinos.sandia.gov/Trilinos11CMakeQuickstart.txt

(*) Disabling the Fortran compiler and all Fortran code:

  To disable the Fortran compiler and all Trilinos code that depends on
  Fortran set:

    -D Trilinos_ENABLE_Fortran:BOOL=OFF

  The user cache variable Trilinos_ENABLE_Fortran is used as a trigger in the
  Trilinos CMake build system to enable Fortran support or not.

  NOTE: The fortran compiler will be disabled automatically by default on
  systems like MS Windows.

  NOTE: Macs do not come with a compatible Fortran compiler by default so you
  must turn off Fortran if you don't have a compatible Fortran compiler.