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
You can cross compile 32 bit binaries on 64 bit target in general by adding 
-m32 to gcc. But this is out of scope of this PR. 
optimize=2 is the option to optimize for your build system. 
Maybe we can remove the if statement here, but I cannot test it. It was just copied from the
original tune section.