title: Adding quotes to argument in C++ preprocessor
body: <p>I'd like to pass the name of an include file as a compiler argument so that I can modify a large number of configuration parameters.  However, my C++ build is via a makefile like process that removes quotes from arguments passed to the compiler and pre-processor.  I was hoping to do something equivalent to</p>
  
  <pre><code>#ifndef FILE_ARG
  // defaults
  #else
  #include "FILE_ARG"
  #endif
  </code></pre>
  
  <p>with my command line including -DFILE_ARG=foo.h.  This of course doesn't work since the preprocessor doesn't translate FILE_ARG.</p>
  
  <p>I've tried </p>
  
  <pre><code>#define QUOTE(x) #x
  #include QUOTE(FILE_ARG)
  </code></pre>
  
  <p>which doesn't work for the same reason.</p>
  
  <p>For scripting reasons, I'd rather do this on the command line than go in and edit an include line in the appropriate routine.  Is there any way?</p>