[Rd] Makevars, cc files in multiple directories

[Saptarshi Guha]

Hello,
In the src folder of my R package I have

a.cc
b.cc
f/g/x.cc

my Makevars.in has

all: $(SHLIB)

upon installing only, a.o and b.o is build and the final dll is  
comprised of a.o and b.o

How can I instruct $(SHLIB) to pick up its source files from all  
subdirectories (or maybe a subset, though here it will be all)
in src ?

Much thanks
Saptarshi