Enable a language (CXX/C/Fortran/etc)
enable_language(<lang> [OPTIONAL] )
This command enables support for the named language in CMake. This is the same as the project command but does not create any of the extra variables that are created by the project command. Example languages are CXX, C, Fortran.
This command must be called in file scope, not in a function call. Furthermore, it must be called in the highest directory common to all targets using the named language directly for compiling sources or indirectly through link dependencies. It is simplest to enable all needed languages in the top-level directory of a project.
OPTIONAL keyword is a placeholder for future implementation and
does not currently work.