mini-os: add BUILD_BUG_ON()

Just copy over what the hypervisor currently uses (comments dropped due
to being of unknown origin; the implementation was done by me anyway).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Drop comments and clarify origin as per Tim's suggestion.

--- a/extras/mini-os/include/lib.h
+++ b/extras/mini-os/include/lib.h
@@ -54,6 +54,15 @@
 #include <xen/event_channel.h>
 #include "gntmap.h"
 
+#if __GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6)
+#define BUILD_BUG_ON(cond) ({ _Static_assert(!(cond), "!(" #cond ")"); })
+#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>