Address GCC8 warning

The bootstrapping allocator needs to perform a memcpy to bypass the
removed move constructors on std::atomic.  This is safe as there is no
concurrency at this point, but GCC is unhappy with this.

This commit moves CI to GCC8 and disables this warning for that line.

src/ds/defines.h

@@ -18,6 +18,12 @@
 #  define SNMALLOC_PURE __attribute__((const))
 #endif
 
+#if !defined(__clang__) && defined(__GNUC__)
+#  if __GNUC__ >= 8
+#    define GCC_VERSION_EIGHT_PLUS
+#  endif
+#endif
+
 #ifndef __has_builtin
 #  define __has_builtin(x) 0
 #endif