* Modify xmkmf so that we can override the imake command invoked via an
environment variable IMAKE_CMD.
* Drop the nbitools/buildlink3.mk file as it's no longer used by either
canna or sj3 for package builds.
Bump the PKGREVISION to 4.
The itools-R6.3 distribution contains imake, X11 configuration files,
makedepend, xmkmf, mkdirhier, imboot, msub, and imdent.
http://www.kitebird.com/imake-book/
nbitools-R6.3 is rearchived with NetBSD-current/xsrc.
Approved by agc, for inputmethod/{canna-*,sj3-*}