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
We shouldn't change the meaning of the flag values since other distros have scripts that will compile Mixxx and they use settings we have published in the past. 

Our release packages are built with ```optimize=9```.