fix #820 : GCC v3.x 32-bits doesn't define 64-bits intrinsic

fix #820 : GCC v3.x 32-bits doesn't define 64-bits intrinsic

resulting in undefined symbol error.
Push the requirement to GCC 4 for now.
Another solution, proposed by @NWilson, is to use __LONG_MAX__ instead.
__LONG_MAX__ is a GCC-specific constant, which value is supposed to depend on underlying target hardware (32/64 bits)
Might be better, but seems also more complex, hence more prone to side effects.
Keeping the simple solution for now (just rely on __GNUC__)