title: Cygwin's cmake does not match for CMAKE_SYSTEM_NAME
body: <p>I'm having a stange behavior on cygwin using cmake.
  I try to check the OS in my CMakeLists.txt but is seems that this particular case does not work...
  Here is an example that raises my problem:</p>
  
  <pre><code>set (FOO "BAR")
  
  message(${CMAKE_SYSTEM_NAME})
  
  if (${CMAKE_SYSTEM_NAME} STREQUAL "CYGWIN")
      message("EQUALS CYGWIN")
  endif()
  
  if (${CMAKE_SYSTEM_NAME} MATCHES "CYGWIN")
      message("MATCHES CYGWIN")
  endif()
  
  if (${FOO} MATCHES "BAR")
      message("MATCHES BAR")
  endif()
  </code></pre>
  
  <p>CMake prints:</p>
  
  <pre><code>CYGWIN
  EQUALS CYGWIN
  MATCHES BAR
  </code></pre>
  
  <p>And not the expected "MATCHES CYGWIN".
  I find this weird that it works for other variables (like FOO here).
  Is there something I am doing wrong?</p>
  
  <p>Configuration:</p>
  
  <ul>
  <li>cmake version 2.8.11.2</li>
  <li>uname CYGWIN_NT-6.1</li>
  </ul>
  
  <p>PS: I also checked with FOO = "CYGWIN", and it does not match neither. It seems that just this particular string does not work with MATCHES...</p>