Mark known feature variables as used

Some features, such as the Fortran wrappers, are enabled by options,
and it may be usable to have those set in a common definition file.

However, as these features are included conditionally based on use,
their option variables may not exist in other projects, giving a CMake
warning for them.

Here we list such variables too. Reading them at the end corresponds
to the Autotools convention of ignoring unknown --enable-xxx options.
Merge pull request #341 from rolk/341_known

Mark known feature variables as used