title: support for arch / arch_only flags in different (macports) gcc versions
body: <p>I'm on OSX snow leopard and using macports for newer versions of gcc.</p>
  
  <p>What is the meaning of the -arch_only compiler flag for gcc? I'm confused as to what it does (I'm aware that "-arch x86_64" makes a 64 bit binary, but that's about it), and why support for it seems to vary between gcc versions.</p>
  
  <p>macports gcc 4.4 supports it, macports 4.6-4.8 don't and give this error:</p>
  
  <pre><code>gcc-mp-4.8: error: unrecognized command line option '-arch_only'
  </code></pre>
  
  <p>Related to this, support for '-arch' seems to vary between versions too. 4.4 supports it, 4.6 doesn't, 4.7 and 4.8 do. </p>