There is no -O4 in gcc, read the source ;)
Remove setting /wd4800 twice for MSVC
Honor cflags set by the user

In case the user is setting CMAKE_CXX_FLAGS, we assume that he does so
intentionally and don't change it. Needed for cross compilation and
packaging.
Remove SSE branches from CMake build system

Feature branches (like SSE) should be done by the preprocessor to allow
cross compiling.
Merge pull request #149 from jspricke/cmake-flags

A couple of CMake fixes, mostly cflags related