title: cmake string token inclusion check
body: <p>In cmake, how can I check if a string token is included in another string?</p>
  
  <p>In my case, I would like to know if the name of the compiler contains the string "Clang" (e.g. "clang", "AppleClang", ...). All I could do so far is:</p>
  
  <pre><code>if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
  ...
  elseif("${CMAKE_CXX_COMPILER_ID}" STREQUAL "AppleClang")
  ...
  </code></pre>
  
  <p>I would like a more flexible approach, like checking for the presence of a substring.  </p>
  
  <p>This is what I could find in the documentation:   </p>
  
  <p>if( MATCHES regex)
  True if the given string or variable’s value matches the given regular expression.<br>
  if( LESS )
  True if the given string or variable’s value is a valid number and less than that on the right.<br>
  if( GREATER )
  True if the given string or variable’s value is a valid number and greater than that on the right.<br>
  if( EQUAL )
  True if the given string or variable’s value is a valid number and equal to that on the right.<br>
  if( STRLESS )
  True if the given string or variable’s value is lexicographically less than the string or variable on the right.<br>
  if( STRGREATER )
  True if the given string or variable’s value is lexicographically greater than the string or variable on the right.<br>
  if( STREQUAL )
  True if the given string or variable’s value is lexicographically equal to the string or variable on the right.  </p>