build: CMake 4.3 build issue with C++ modules in clang fixed

Author: mpusz

src/CMakeLists.txt

@@ -112,6 +112,12 @@ if(MP_UNITS_BUILD_CXX_MODULES)
         message(FATAL_ERROR "CMake versions before 3.29 do not support C++ modules properly")
     endif()
     set(MP_UNITS_TARGET_SCOPE "PUBLIC")
+    if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
+        # Workaround for CMake 4.3 + Clang 20/21 incompatibility:
+        # CMake 4.3 generates both `--precompile` and `-c` for @synth targets (CMake bug);
+        # Clang 20/21 reports this as -Wunused-command-line-argument (now a default-on warning)
+        add_compile_options(-Wno-error=unused-command-line-argument)
+    endif()
 else()
     set(MP_UNITS_TARGET_SCOPE "INTERFACE")
 endif()
@@ -124,13 +130,11 @@ if(CMAKE_CXX_MODULE_STD)
         message(FATAL_ERROR "`import std;` requires at least C++23")
     endif()
     if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
-        # Workaround for two CMake 4.3 + Clang 21 incompatibilities:
-        # 1. CMake 4.3 generates both `--precompile` and `-c` for @synth targets (CMake bug);
-        #    Clang 21 reports this as -Wunused-command-line-argument (now a default-on warning)
-        # 2. Clang 21 reports `export module std;` in libc++'s std.cppm as
-        #    -Wreserved-module-identifier (now a default-on warning), because CMake compiles
-        #    the stdlib module with the project's compiler flags
-        add_compile_options(-Wno-error=unused-command-line-argument -Wno-error=reserved-module-identifier)
+        # Workaround for CMake 4.3 + Clang 21 incompatibility:
+        # Clang 21 reports `export module std;` in libc++'s std.cppm as
+        # -Wreserved-module-identifier (now a default-on warning), because CMake compiles
+        # the stdlib module with the project's compiler flags
+        add_compile_options(-Wno-error=reserved-module-identifier)
     endif()
 endif()