xen: introduce a C99 headers check

xen: introduce a C99 headers check

Introduce a C99 headers check, for non-ANSI compliant headers: 9pfs.h
and pvcalls.h.

In addition to the usual -include stdint.h, also add -include string.h
to the C99 check to get the declaration of memcpy and size_t.

For the same reason, also add -include cstring to the C++ check when
necessary.

Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
CC: JBeulich@suse.com