Fix #867: cmake doesn't issue `-isystem` for versioned gcc

CMake purposefully disables the use of the `-isystem` flag on Apple
platforms, but it's overly blunt with the detection.  Apple's compilers
have supported the flag since at least 10.4.  Let's force the switch to
be on when gcc/g++ is detected on an Apple platform to reduce the
warnings out of the msgpack-related bits.