CMake: Add the list of possible values for some config options

Signed-off-by: Paul Cercueil <paul@crapouillou.net>

CMakeLists.txt

@@ -3,8 +3,9 @@ project(OpenMW)
 # If the user doesn't supply a CMAKE_BUILD_TYPE via command line, choose one for them.
 IF(NOT CMAKE_BUILD_TYPE)
     SET(CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING
-        "Choose the type of build, options are: Debug Release RelWithDebInfo MinSizeRel."
+        "Choose the type of build, options are: None(CMAKE_CXX_FLAGS or CMAKE_C_FLAGS used) Debug Release RelWithDebInfo MinSizeRel."
         FORCE)
+    set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS None Debug Release RelWithDebInfo MinSizeRel)
 ENDIF()
 
 if (APPLE)
@@ -175,6 +176,7 @@ endif()
 # Dependencies
 if (USE_QT)
     set(DESIRED_QT_VERSION 4 CACHE STRING "The QT version OpenMW should use (4 or 5)")
+    set_property(CACHE DESIRED_QT_VERSION PROPERTY STRINGS 4 5)
     message(STATUS "Using Qt${DESIRED_QT_VERSION}")
 
     if (DESIRED_QT_VERSION MATCHES 4)