Automatically check xen's public headers for C++ pitfalls.

Automatically check xen's public headers for C++ pitfalls.

Add a check, like the existing check for non-ANSI C in the public
headers, that runs the public headers through a C++ compiler to
flag non-C++-friendly constructs.

Unlike the ANSI C check, we accept GCC-isms (gnu++98), and we also
check various tools-only headers.

Also tidy up the runes for these checks to be a bit more readable.

io/ring.h uses 'private' as a field name, which is a keyword in C++;
this patch doesn't change that.  Instead the check works around it.

Reported-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Signed-off-by: Tim Deegan <tim@xen.org>
Acked-by: Jan Beulich <JBeulich@suse.com>
Tested-by: Don Slutz <dslutz@verizon.com>