title: cmake: add defines of data types handled as string
body: <p>Standard C
  I need to add data  types to project because GNU do not understand some C51 data types. Example need a BYTE types as:</p>
  
  <pre><code>#define BYTE unsigned char
  </code></pre>
  
  <p>Have tried following examples:</p>
  
  <pre><code>add_definitions(-DBYTE=\"unsigned char\")
  add_definitions(-DBYTE="unsigned short")
  add_definitions(-DBYTE="\"unsigned long\"")
  </code></pre>
  
  <p>Some other ideas?</p>
  
  <p>thanks :-)</p>