Fix option name to match name in use in conditional.

A mistake was made when renaming the option (not renaming it everywhere).

This did not create any trouble for me, perhaps since I did not use ccmake to set my options but give them in a cache preload file with the -C option to cmake. Anyone who tried to activate the option in ccmake would probably not see the wished-for effect, anyway.

Also added a doc string that specifies minimum gcc version.