Add AVX2 version of od_bin_fdct8x8.

This PR contains the previous PR which is still pending. I think GitHub will automatically figure out that part of it is merged once we merge the other one. I was running into LLVM crashes trying to set __AVX2__ manually, so I changed the build system to actually build libraries with separate CFLAGS. That seems cleaner anyway. In addition I added DEBUG versions of the libraries for dcttest (asserts and overflow checks enabled). I will SSE2 and SSE4.1 variants derived from the AVX2 code. I sent an email to Intel for the integer matrix transpose. I couldn't find a good public source for an integer version and my algorithm doesn't feel optimal to me.