[cmake] properly check for defined variable

cmake/modules/DunePythonInstallPackage.cmake

@@ -277,7 +277,7 @@ function(dune_python_install_package)
     set(_cmake_flags "")
     foreach(_dep ${ProjectName} ${ALL_DEPENDENCIES})
       dune_module_to_uppercase(upper ${_dep})
-      if(NOT ${HAVE_${upper}} STREQUAL "")
+      if(DEFINED ${HAVE_${upper}})
         list(APPEND _cmake_flags "HAVE_${upper}:=${HAVE_${upper}}")
       endif()
     endforeach()