title: Define preprocessor macros in a .config file
body: <p>I'm coding in C using CMake and I would like to define sobre preprocessor macros in a separate file. The purpose of this is to have a main configuration file for my apps.</p>
  
  <p>I was tinking in doing something similar to the kernel .config files, but I didn't find how to pass a file with definitions to the compiler. I know the option -D of gcc to define a single macro, but it does not accept files as imput. I also know the ADD_DEFINITIONS utility of CMake but the usage but it is also expected to be used with a certain amount of macros, not a file.</p>
  
  <p>Summarizing, I would like to define a file .config with macro definitions:</p>
  
  <pre><code>CONFIG_MACRO_A=y
  CONFIG_MACRO_B=y
  # CONFIG_MACRO_C is not set
  CONFIG_MACRO_C=y
  </code></pre>
  
  <p>And employ each line as a preprocessor macro in compilation time.</p>
  
  <p>Thank you so much for the help.</p>