improved build system.

* added a USE_SSE CMake option (default is ON): when set to ON, the APPLESEED_USE_SSE preprocessor symbol is defined and the -msse and -msse2 compilation flags (or their equivalents on Windows) are added.
* removed the -march=native compilation flag that was added in 23b9234fa30250fe6b408085197042c060ee0eaa for Linux and OS X platforms.
* the version string returned by foundation::Appleseed::get_lib_version() now states whether appleseed was built with SSE instruction sets enabled.
* miscellaneous fixes, tweaks and cleanups.