Fixing libswipl symbol leakage

Hey folks! For anyone linking against, just so you’re aware, I’m updating the swipl build process to hide library-internal symbols by default. This won’t affect you as long as you’re being well-behaved and only using functions declared in SWI-Prolog.h or SWI-Stream.h, but if you’ve been calling any internal functions whose prototypes aren’t declared in one of those two files, now is the time to let me know!