Covert gcc arguments to CMake file - c++

Question

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.txt
cmake_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 done
CMake 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.

Answer 1

The trick is execute_process.
execute_process(COMMAND uname -s
OUTPUT_VARIABLE uname_output
OUTPUT_STRIP_TRAILING_WHITESPACE
)
add_executable(cli_arch cli_arch.c arch.c)
target_compile_definitions(cli_arch
PRIVATE "-DARCH=\"${uname_output}\""
)
I had to use uname -s since at least on my system, uname -a has a # which gcc refuses to accept in a macro via the command line.
Edit, since you changed your question. You're passing ARCH to cmake but not doing anything will it. You can pass the argument along using the same target_compile_definitions line I have above.
$ cmake -DARCH2=foo .
$ make
$ ./a
Linux
foo
The relevant part of the new CMakeLists.txt looks like this:
target_compile_definitions(a
PRIVATE "-DARCH=\"${uname_output}\""
PRIVATE "-DARCH2=\"${ARCH2}\""
)