ctest: Disable CMake Cygwin legacy warning in dashboard client script

In ctest -S mode we load the CTestScriptMode module to give the
configuration scripts access to the host system information.
CYGWIN.cmake warns unless we explicitly tell it not to warn
since there is no chance for a cmake_minimum_required(VERSION)
to be called.

Source/CTest/cmCTestScriptHandler.cxx

@@ -421,6 +421,10 @@ int cmCTestScriptHandler::ReadInScript(const std::string& total_script_arg)
     this->Makefile->AddDefinition("CTEST_SCRIPT_ARG", script_arg.c_str());
     }
 
+#if defined(__CYGWIN__)
+  this->Makefile->AddDefinition("CMAKE_LEGACY_CYGWIN_WIN32", "0");
+#endif
+
   // always add a function blocker to update the elapsed time
   cmCTestScriptFunctionBlocker *f = new cmCTestScriptFunctionBlocker();
   f->CTestScriptHandler = this;