[CMake] Adjust Fortran check to how Eigen does it.

We got the workaround_9220 code from Eigen. Now we
use the code as the Eigen project does, see for
example their file lapack/CMakeLists.txt.
This fixes the Fortran detection with Visual Studio
2015 CTP 6.

cmake/modules/DuneMacros.cmake

@@ -569,6 +569,9 @@ macro(dune_project)
   workaround_9220(Fortran Fortran_Works)
   if(Fortran_Works)
     enable_language(Fortran OPTIONAL)
+    if(NOT CMAKE_Fortran_COMPILER)
+      set(Fortran_Works OFF)
+    endif()
   endif(Fortran_Works)
 
   option(DUNE_USE_ONLY_STATIC_LIBS "If set to ON, we will force static linkage everywhere" OFF)