Compile intrinsic functions with appropriate flags in gcc.

- Remove -march=native as it's no longer necessary for intrinsics to work.
  Closes #77.

- I couldn't test altivec or sse4.1, but sse4.1 compiles so I expect it
  to work.
Tidy up the Makefile.
Add missing includes.
Add sse2 implementaton for sad_8bit_4x4.
Add AVX2 implementations for sad_8bit_ 8x8, 16x16 and 32x32.