Log Message:
LIBTOOL_OVERRIDE generally doesn't need to be specified anymore... just
set OVERRIDE_DIRDEPTH to find any libtool scripts deeper in the WRKSRC
tree unless they're named something other than "libtool".

SHLIBTOOL_OVERRIDE generally doesn't need to be specified either -- just
define it to the empty list and shlibtool-override will look for libtool
scripts.