Fix: Don't override parent's BOOST_COMPONENTS when used as submodule

When FC is used as a submodule in hive, the parent project defines
BOOST_COMPONENTS with all needed components (including program_options).

The previous code unconditionally cleared and reset BOOST_COMPONENTS,
which caused link failures due to missing boost::program_options.

Now FC only sets its own BOOST_COMPONENTS when building standalone
(when no parent has defined it).