Add CYGWIN support for DLL library check

(from https://github.com/libsdl-org/SDL_image/pull/747)

cmake/macros.cmake

@@ -277,7 +277,7 @@ function(target_get_dynamic_library DEST TARGET)
   endif()
   if(result)
     string(TOLOWER "${result}" result_lower)
-    if(WIN32 OR OS2)
+    if(WIN32 OR OS2 OR CYGWIN)
       if(NOT result_lower MATCHES ".*dll")
         message(FATAL_ERROR "\"${result}\" is not a .dll library")
       endif()