No option required for BUILD_SHARED_LIBS

build.sh

#!/usr/bin/env bash

cmake -GNinja -Bbuild_static .
cmake -GNinja -DBUILD_SHARED_LIBS=ON -Bbuild_shared .
ninja -C build_static
ninja -C build_shared

ls -al build_static/*test*
ls -al build_shared/*test*

CMakeLists.txt

cmake_minimum_required(VERSION 3.21)
project(Test LANGUAGES CXX)
add_library(test lib.cpp lib.h)

lib.cpp

#include <iostream>

void a_func()
{
    std::cout << "Hello" << std::endl;
}

lib.h

#pragma once

extern void a_func();

log.txt

-- The CXX compiler identification is AppleClang 13.0.0.13000029
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /Applications/Xcode12.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /tmp/test/build_static
-- The CXX compiler identification is AppleClang 13.0.0.13000029
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /Applications/Xcode12.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /tmp/test/build_shared
ninja: Entering directory `build_static'
[1/2] Building CXX object CMakeFiles/test.dir/lib.cpp.o
[2/2] Linking CXX static library libtest.a
ninja: Entering directory `build_shared'
[1/2] Building CXX object CMakeFiles/test.dir/lib.cpp.o
[2/2] Linking CXX shared library libtest.dylib
-rw-r--r--  1 mark  wheel  14824 13 Jan 20:05 build_static/libtest.a
-rwxr-xr-x  1 mark  wheel  56168 13 Jan 20:05 build_shared/libtest.dylib