linking druntime in when C calls D and C implements main()

[Carl Sturtivant]

If I separately compile dfunc.d containing an exern(C) function, 
and cmain.c containing void main() that calls that function 
(after calling rt_init() to initialize druntime) and then ask dmd 
to link the resulting pair of objects into an executable, the 
linker complains that various symbols (including _rt_init) are 
not present, suggesting that (quite reasonably) dmd does not 
place druntime inside an arbitrary d object file. How to I ask 
dmd to link in druntime so as to get a working executable in this 
situation exemplifying a C application calling a D library 
function with C linkage?