use get_python_lib again, but adding parenthesis for print. this syntax works with python 2.7 and 3.4

CMakeLists.txt

@@ -6,7 +6,7 @@ project(open62541 C)
 FIND_PACKAGE(PythonInterp REQUIRED)
 
 # Find Python-lxml
-execute_process ( COMMAND ${PYTHON_EXECUTABLE} -c "import site; site.getsitepackages()" OUTPUT_VARIABLE PYTHON_SITE_PACKAGES OUTPUT_STRIP_TRAILING_WHITESPACE)
+execute_process ( COMMAND ${PYTHON_EXECUTABLE} -c "from distutils.sysconfig import get_python_lib; print(get_python_lib())" OUTPUT_VARIABLE PYTHON_SITE_PACKAGES OUTPUT_STRIP_TRAILING_WHITESPACE)
 if(NOT EXISTS "${PYTHON_SITE_PACKAGES}/lxml")
     message( FATAL_ERROR "Python-lxml is not installed.")
 endif()