[Rd] forcing the use of the c++ linker with c++ and fortran sources

[Steve Guerrero]

I'm trying to build a package which has both gfortran and c++ code.

In my installation of R, the R CMD INSTALL process compiles each c++ and 
gfortran code with the correct compiler, but performs the link step with 
"gfortran" and this creates undefined symbols at load time.

How does one override/force the use of a different command in the link 
step?  Is there an appropriate entry in Makevars that will do the trick?

Thanks,
-sg