Check presence of CC/CXX/FC compilers

[adrien-cotte]

Since MPI requirement was removed, there is no mechanism to check the presence of compilers.

$ cmake -DWI4MPI_COMPILER=intel ..
[...]
-- Configuring done
-- Generating done

$ make
[...]
make: *** [all] Error 2

$ icc
icc: command not found

After this patch, configuration releases an error :

$ cmake -DWI4MPI_COMPILER=intel ..
[...]
CMake Error at CMakeLists.txt:116 (message):
  C compiler not found in PATH!

      Current value of CC is: 


-- Configuring incomplete, errors occurred!
[...]

[adrien-cotte]

Fix #31 too.