[Rd] LinkingTo self

[Romain Francois]

Hello, 

Shipping header files for a package in inst/include and let other packages use it with LinkingTo is popular. 

Unfortunately for the package itself, we still need to use something like :

PKG_CPPFLAGS+=-I../inst/include/

in the Makevars and Makevars.win files. 

Could this become automatic ? 

So if a package has src/ and inst/include/ can we have it so that a -I flag is set accordingly. 

Would anyone be interested in reviewing a patch against R-devel for such feature ?

Romain