CMAKE_COMPILER_IS_GNUCXX and CMAKE_CXX_COMPILER_ID are empty

Question

I am currently playing around with CMake and want to detect the compiler and the compiler version. My current CMakeLists.txt looks as follows:

cmake_minimum_required (VERSION 2.6)

set (PROJECT "a_tour_of_c++")
set (GNUCXX_MINIMUM_VERSION "4.8")
set (CXX_STANDARD "c++11")

message ("${CMAKE_CXX_COMPILER}")       # C:/dev/MinGW/bin/g++.exe
message ("${CMAKE_CXX_COMPILER_ID}")    # EMPTY
message ("${CMAKE_COMPILER_IS_GNUCXX}") # EMPTY

if (CMAKE_COMPILER_IS_GNUCXX)
  if (CMAKE_CXX_COMPILER_VERSION VERSION_LESS GNUCXX_MINIMUM_VERSION)
    message (FATAL_ERROR "GCC version must be at least ${GNUCXX_MINIMUM_VERSION}!")
  endif()
else()
  message (FATAL_ERROR "Invalid compiler!")
endif()

set (BUILD_DIR build)
set (HEADER_DIR include)
set (SOURCE_DIR src)
set (MAIN_FILE ${SOURCE_DIR}/main.cc)

set (CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=${CXX_STANDARD}")
project ($(PROJECT))
include_directories (${HEADER_DIR})
add_executable (${BUILD_DIR}/${PROJECT} ${MAIN_FILE})

The file is fine and I can sucessfully clean, build and run my cli application with this file (if I comment out the else-branch of the compiler check).

The problem is, that both CMAKE_CXX_COMPILER_ID and CMAKE_COMPILER_IS_GNUCXX are empty (CMAKE_CXX_COMPILER is not).

I am using a Microsoft Windows 8 (6.3.9600) operating system and MinGW, more detailed the following tools.

As you can in the screenshot, I use NetBeans IDE to simplify the whole build progress.

So my questions are:

1. What are possible reasons, that the mentioned variables are empty?
2. Does any workaround exist for my problem?

And please let me know, if you see any other problems with my (simple) CMakeLists.txt file.