Change CFLAGS to '-march=native' by default

Hi all, I noticed that `setup.py` currently only checks whether the compiler will accept the `-msse2` flag, but SSE2 is pretty ancient now, and most people's CPUs will support a bunch of newer instruction sets such as SSE4 and AVX. Is there any reason not to try setting `-march=native` first, given that this entails `-msse2` along with whatever other optimization flags GCC deems appropriate? I've read somewhere that very old versions of GCC (< 4.2) don't support this flag, but there's already a mechanism to check whether the compiler will choke on it, and we can always try `-msse2` next if this happens.