title: XCode using schemes to modify build settings
body: <p>I am trying to use Schemes in XCode to modify some flags in build settings.</p>
  
  <p>My first attempt was to add environment variables in schemes, but this seems to apply only at run time, not at compile time.</p>
  
  <p>My second attempt was to add a script setting an environment variable as a pre-action. This doesn't seem to work either.</p>
  
  <p>I could solve this by duplicating targets, but that is makes it hard to keep build settings in sync. I would just like to use some conditional macros in my code.</p>