[Rd] Compatibility with R 2.15.x: Makefile for (non-Sweave) vignettes in vignettes/?

[Henrik Bengtsson]

In R 3.1.0 (~April 2014), support for vignettes in inst/doc/ will go
away (and probably much sooner for CRAN submission), e.g.

checking for old-style vignette sources ... NOTE
Vignette sources only in ‘inst/doc’:
‘R.devices-overview.tex.rsp’
A ‘vignettes’ directory is required as from R 3.1.0
and these will not be indexed nor checked

I've been sticking with inst/doc/ for backward compatible reasons so
that I can use a "fallback" inst/doc/Makefile for building
*non*-Sweave vignettes also under R 2.15.x.   AFAIK, it is not
possible to put a Makefile under vignettes/, i.e. it is not possible
to build non-Sweave vignette under vignettes/.

For backward compatible reason, I'd like to keep distributing my
packages on CRAN with non-Sweave vignettes available for R (>=
2.15.0), not just R (>= 3.0.0).  Does anyone know an alternative
approach?

/Henrik