mini-os: add BUILD_BUG_ON()

Just copy over what the hypervisor currently uses.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/extras/mini-os/include/lib.h
+++ b/extras/mini-os/include/lib.h
@@ -54,6 +54,20 @@
 #include <xen/event_channel.h>
 #include "gntmap.h"
 
+#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
+/* Force a compilation error if condition is true */
+#define BUILD_BUG_ON(cond) ({ _Static_assert(!(cond), "!(" #cond ")"); })
+/* Force a compilation error if condition is true, but also produce a
+   result (of value 0 and type size_t), so the expression can be used
+   e.g. in a structure initializer (or where-ever else comma expressions
+   aren't permitted). */
+#define BUILD_BUG_ON_ZERO(cond) \
+    sizeof(struct { _Static_assert(!(cond), "!(" #cond ")"); })
+#else
+#define BUILD_BUG_ON_ZERO(cond) sizeof(struct { int:-!!(cond); })
+#define BUILD_BUG_ON(cond) ((void)BUILD_BUG_ON_ZERO(cond))
+#endif
+
 #ifdef HAVE_LIBC
 #include <sys/queue.h>
 #include <stdio.h>