title: Why cmake shows message when it should not?
body: <p>Fltk is found, but test not passed, why?</p>
  <h1>define a simple macro</h1>
  <pre><code>macro(assert TEST COMMENT)
    message(${TEST})
    if(NOT ${TEST})
      message(&quot;Assertion failed: ${COMMENT}&quot;)
    endif()
  endmacro()
  
  # use the macro
  find_library(FLTK_LIB fltk)
  assert(${FLTK_LIB} &quot;Unable to find library fltk&quot;)
  </code></pre>
  <p>Output:</p>
  <pre><code>/usr/lib/x86_64-linux-gnu/libfltk.so
  Assertion failed: Unable to find library fltk
  </code></pre>