title: How to test if CMake found a library with find_library
body: <p>I find the library with the <code>find_library</code> function</p>
  <pre><code>find_library(MY_LIB lib PATHS ${MY_PATH})
  </code></pre>
  <p>If the library is found, <code>${MY_LIB}</code> will point to the correct location.
  If the library is not found <code>${MY_LIB}</code> will be <code>MY_LIB-NOTFOUND</code>.</p>
  <p>But how do I test this?</p>
  <pre><code>if(${MY_LIB} EQUAL 'MY_LIB-NOTFOUND') 
      ...
  endif()
  </code></pre>
  <p>always evaluates to false.</p>