Print clear CMake error if six python module is not installed

CMakeLists.txt

@@ -6,6 +6,12 @@ string(TOLOWER "${CMAKE_BUILD_TYPE}" BUILD_TYPE_LOWER_CASE)
 
 set(CMAKE_MODULE_PATH "${PROJECT_SOURCE_DIR}/tools/cmake")
 find_package(PythonInterp REQUIRED)
+# Verify that the six python module is correctly installed 
+execute_process(COMMAND "${PYTHON_EXECUTABLE}" "-c"  "import six"
+                RESULT_VARIABLE import_six_return OUTPUT_QUIET ERROR_QUIET)
+if(NOT ${import_six_return} EQUAL 0)
+   message(FATAL_ERROR "Python interpreter found, but required six python module is not installed.")
+endif()
 find_package(Git)
 include(AssignSourceGroup)