We'll use this to improve the definition of container_of(). Let's define
it if it does not exist. We can rely on __builtin_offsetof() on recent
enough compilers.
This is ebtree commit 1ea273e60832b98f552b9dbd013e6c2b32113aa5.
Signed-off-by: Willy Tarreau <w@1wt.eu>
This is ebtree commit 69b2ef57a8ce321e8de84486182012c954380401.