Optimize gcc

This reduces the "optimize" scons option too three and integrates the tuning option.
optimize: 
Set to:
  1 portable-binary: sse2 CPU (>= Pentium 4)
  2 native-binary: optimized exclusive for the CPU of this system
  3 legacy-binary: pure i386 code

Tested on Linux only 

https://bugs.launchpad.net/mixxx/+bug/1405678
```machine_is_64bit``` should maybe have been named ```target_machine_is_64bit``` for clarity .. as @daschuer mentions it's the target machine (architecture + bitwidth -- i.e. powerpc64 vs powerpc, x86_64 vs. i686, arm64 vs. armhf)