Dial down gcc/clang optimization to O2

Fixes #157.
Reduce optimization level from -O3 to -O2 per Debian guidance.
Remove some dead compilation flags, including the -msse2 instruction set selection, for gcc & clang.
Yes. I wanted to leave it in place though because it took me forever to figure out the magic CMAKE_CL_64 flag that can be used to distinguish 64-bit Visual Studio builds from 32-bit ones. This way if we need to make that distinction again it will be easy to do (I think).