cmake force g++ gcc version to use

$cmake -DCMAKE_CXX_COMPILER=g++-8  -DCMAKE_CC_COMPILER=gcc-8 ..

This command affec to next variables into CMakeLists.txt file
CMAKE_CXX_COMPILER_VERSION
CMAKE_CXX_COMPILER_ID


#Next lines doesn't works for me on this time.

set(CMAKE_C_COMPILER /usr/bin/gcc-8)
set(CMAKE_CXX_COMPILER /usr/bin/g++-8)