Provide compatibility wrappers for system (i.e. distribution-provided)
ConcurrencyKit on i386. Unlike bundled ConcurrencyKit, older versions
that may be provided by distributions do not support CK_USE_CC_BUILTINS,
which makes implementing 64-bit atomics impossible on x86 (32-bit).
Detect if 64-bit atomics are not provided at compile stage and if so,
resort to GCC builtin implementations.
