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.

This should really have been a part of #330, but wasn't discovered before due to insufficient across-the-board-testing.