tests: skip C++ tests if we can't compile a simple program

tests: skip C++ tests if we can't compile a simple program

There are possible conflicts between gnulib replacement functions (in
<stdio.h>) and their C++ wrappers (in <stream>).  Trying to address
these in configure seems too hard, and I don't know how to fix the issue
in gnulib.  Cowardly avoid the problem by skipping C++ tests when this
happens.
Reported by Stefano Lattarini.
http://lists.gnu.org/archive/html/bug-bison/2013-06/msg00001.html

* tests/atlocal.in (BISON_CXX_WORKS): Also set it to "skip" if we can't
compile a simple program using <stream>.
* tests/local.at: Comment changes.