how to set CMAKE_CXX_COMPILER ?

I have two gcc in path , how can I use the second?

Now I have to fix
mbed-os\tools\cmake\toolchains\GCC_ARM.cmake

set(CMAKE_ASM_COMPILER "arm-none-eabi-gcc")
set(CMAKE_C_COMPILER "arm-none-eabi-gcc")
set(CMAKE_CXX_COMPILER "arm-none-eabi-g++")

to

# set(CMAKE_ASM_COMPILER "arm-none-eabi-gcc")
# set(CMAKE_C_COMPILER "arm-none-eabi-gcc")
# set(CMAKE_CXX_COMPILER "arm-none-eabi-g++")

then use

cmake -D CMAKE_CXX_COMPILER=xxx -D CMAKE_C_COMPILER=xxx ?

Start with mbed-os\tools\cmake\toolchains\GCC_ARM.cmake and the CMake command shown in the issue. Determine whether compiler selection is intended to be passed through CMake variables or requires toolchain-file changes; done means documenting or supporting a clear way to select the second GCC installation.

Written by the indexing model from the issue text.