games/scummvm: Partially revert configure patch

The use of "$(prefix)" in the configure file seems not to be
understand. On FreeBSD, it's ignored but it actually causes failures
on DragonFly. I suspect these flags aren't actually used.

If it turns out they are used, and support for non-standard PREFIX
(not /usr/local) is desired, the REINPLACE_CMD macro can adjust
/usr/local with in a post-patch target.