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

.gitignore

@@ -285,8 +285,7 @@ xen/arch/*/efi/boot.c
 xen/arch/*/efi/compat.c
 xen/arch/*/efi/efi.h
 xen/arch/*/efi/runtime.c
-xen/include/headers.chk
-xen/include/headers++.chk
+xen/include/headers*.chk
 xen/include/asm
 xen/include/asm-*/asm-offsets.h
 xen/include/asm-x86/cpuid-autogen.h