This patch does the first minor steps towards TR 29113: It accepts the 
OPTIONAL attribute in procedures with C binding. As gfortran already 
passes absent arguments a NULL pointer, there is no changed needed, 
except in the diagnostics part. Additionally, it is a feature also 
supported by other compilers.

Additionally, this patch adds the option -std=f2008tr (tr = Technical 
Report(s)), which is supposed to cover Fortran 2008 and the two 
technical reports, TR 29113 (further interop with C) and the TR about 
enhanced coarray support.

I think it would be nice if TYPE(*) and DIMENSION(..) could also be 
implemented during the 4.7 development period as they are useful for C 
interoperability ("void *buffer") - and in particular for MPI 3; 
additionally, they do not seem to be that difficult to implement. 
However, I do not plan to work on them in the near future.

Build and regtested on x86-64-linux
OK for the trunk?

The implementation status will be tracked at 
http://gcc.gnu.org/wiki/TR29113Status ;-)

Tobias

PS: I included in the patch also a patchlet for IMPORT, cf. Mikael's 
comment at http://gcc.gnu.org/ml/fortran/2011-05/msg00005.html; with 
that patch, "sym" is always set to NULL when entering the loop for the 
next symbol. I think it is not really needed as "gfc_current_ns->parent" 
should be always "!= NULL", but maybe one manages have it NULL.