Ref D8855: CMake/macOS: Remove _LIBPATH, avoid link_directories.

Unix and Apple platform files use find_package(OpenSubdiv) which sets
OPENSUBDIV_INCLUDE_DIR as an advanced variable, as well as
OPENSUBDIV_INCLUDE_DIRS which should be used usually.
Windows sets OPENSUBDIV_INCLUDE_DIR which is used by the rest
of the code.

This patch renames it to _DIRS everywhere, for it to be like other
similar variables.