RFC: add some static probes to libstdc++

[Tom Tromey]

Tom> 2013-02-27  Tom Tromey  <tromey@redhat.com>
Tom> 	* libsupc++/unwind-cxx.h: Include sys/sdt.h if detected.
Tom> 	(PROBE2): New macro.
Tom> 	* libsupc++/eh_throw.cc (__cxa_throw, __cxa_rethrow): Add probe.
Tom> 	* libsupc++/eh_catch.cc (__cxa_begin_catch): Add probe.
Tom> 	* configure.ac: Check for sys/sdt.h.
Tom> 	* configure, config.h.in: Rebuild.

FWIW the gdb patches relying on this have all been submitted.
They are pending comments, but also this patch going in.

In particular:

Using the probes in gdb:
    http://sourceware.org/ml/gdb-patches/2013-03/msg00242.html

Using the probes to implement the $_exception convenience variable:
    http://sourceware.org/ml/gdb-patches/2013-03/msg00243.html

Using the probes to implement filtering for exception catchpoints:
    http://sourceware.org/ml/gdb-patches/2013-03/msg00245.html

Tom