fixed bug in CMake script checking for C++11-compliant compiler for CMake<2.8

- older CMake versions do not know the CMAKE_CXX_COMPILER_VERSION variable, which introduced a bug in the version check for C++11 compliance
- added a compatability variable for older CMake versions into CMakeCompatibility.cmake
- fixes eudaq/eudaq#45
made error message on failed C++11 compiler check fatal and more verbose
changed CMake print script to use routine of CMakeCompatability.cmake for compiler version parsing