FPC can link against dylibs, it's what the rtl does since Darwin (the OS core of MacOSX) has no kernel interface like linux which can be used without libc.

Regarding MacOSX and SDL have a look at:
http://www.freepascal.org/wiki/index...pecific_issues

If you've question about it, ask on the fpc-mailing-lists, it will be probably read by someone who knows more about MacOSX than me