Covert gcc arguments to CMake file

So I am trying to learn CMake and I want to convert the following

gcc "-DARCH=\"`uname -a`\"" cli_arch.c arch.c 

to a CMakeLists.txt file.

This is what I got so far:

Running cmake -DCMAKE_C_COMPILER=/usr/bin/gcc "-DARCH=\"`uname -a`\"" ..

// CMakeLists.txtcmake_minimum_required(VERSION 2.8)project(cli_arch)add_executable(cli_arch cli_arch.c arch.c)

The result I am getting is:

-- The C compiler identification is AppleClang 11.0.3.11030032-- The CXX compiler identification is AppleClang 11.0.3.11030032-- Check for working C compiler: /usr/bin/gcc-- Check for working C compiler: /usr/bin/gcc -- works-- Detecting C compiler ABI info-- Detecting C compiler ABI info - done-- Detecting C compile features-- Detecting C compile features - done-- Check for working CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++-- Check for working CXX compiler: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ -- works-- Detecting CXX compiler ABI info-- Detecting CXX compiler ABI info - done-- Detecting CXX compile features-- Detecting CXX compile features - done-- Configuring done-- Generating doneCMake Warning:  Manually-specified variables were not used by the project:    ARCH

I am not understanding how to pass the arguments "-DARCH=\"`uname -a`\"" to CMake.