This patch is a teaser: It adds some preparation for coarray 
communication to the trunk, but it doesn't yet added the library calls. 
(In particular, note the "0 &&" in the conditions, which add the 
intrinsics.)

The idea is that coindexed variables are wrapped in the intrinsic 
function caf_get(expr) and assignments to coindexed variables are 
wrapped into the subroutine caf_send(LHS, RHS). Both intrinsic 
procedures are only internally reachable. The intrinsic procedures are 
then converted into library calls in trans-intrinsic.c. (See fortran-caf 
branch.)

I am not yet submitting the the trans-intrinsic.c part as I still want 
to restructure the current version – and I am also unsure how exactly I 
should handle allocatable components of coarrays. But as this part 
should be ready, I thought that I already submit it, reducing the 
differences between the branch and the trunk. However, I can also hold 
off this patch until it is actually used.

Build on x86-64-gnu-linux.
OK for the trunk?

Tobias