Dejagnu: use -isystem to include system header files.

[Richard Earnshaw]

> > I think the gcc/include directory must be added implicitly 
> from the -B
> > option.  It would appear that these add -isystem type include
> > directories, so it might be just a matter of ordering the -B and
> > -isystem options appropriately.
> 
> But - how would this help in the situation where -ansi and -pedantic 
> have been specified as well.  In those cases we do not want 
> to get the 
> limits.h file from newlib.

The issue wasn't really about picking up gcc/include/limits.h over
newlib/include/limits.h, it was about how we processed the newlib
version when in strict ANSI mode.  That should be handled correctly when
we use -isystem (because that relaxes the rules).  So I think we should
try and get the search order back to what it was before, but when still
using -isystem.

R.