Log Message:
boehm-gc: add upstream commit for libtool version

In 7.6.2, upstream intentionally but badly-advised bumped their
library major version number. In 7.6.4, they'll undo that change.
Instead of bumping twice in pkgsrc, follow their lead and get the
change in now.

See https://github.com/ivmai/bdwgc/issues/197

Bump PKGREVISION.