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 let you specify a bunch of directory names on the command line
to *all* packages, without getting annoying warnings that they are
defined but not used.
