Add link to reason for gcc behavior change.
@@ -113,6 +113,7 @@ int Test::Main()
std::atomic<vespamalloc::TaggedPtr> taggedPtr;
ASSERT_EQUAL(16u, sizeof(vespamalloc::TaggedPtr));
#if __GNUC__ < 7
+ // See https://gcc.gnu.org/ml/gcc-patches/2017-01/msg02344.html for background
ASSERT_TRUE(taggedPtr.is_lock_free());
#else
ASSERT_FALSE(taggedPtr.is_lock_free());
