build: C++23 enabled for test_package when import std is ON

Author: mpusz

test_package/CMakeLists.txt

@@ -28,11 +28,13 @@ message(STATUS "MP_UNITS_BUILD_CXX_MODULES: ${MP_UNITS_BUILD_CXX_MODULES}")
 find_package(mp-units REQUIRED)
 
 add_executable(test_package-headers test_package.cpp)
-target_compile_features(test_package-headers PRIVATE cxx_std_20)
+target_compile_features(test_package-headers PRIVATE $<IF:$<BOOL:${CMAKE_CXX_MODULE_STD}>,cxx_std_23,cxx_std_20>)
 target_link_libraries(test_package-headers PRIVATE mp-units::mp-units)
 
 if(MP_UNITS_BUILD_CXX_MODULES)
     add_executable(test_package test_package.cpp)
-    target_compile_features(test_package PRIVATE cxx_std_20)
+    target_compile_features(test_package PRIVATE $<IF:$<BOOL:${CMAKE_CXX_MODULE_STD}>,cxx_std_23,cxx_std_20>)
+    # TODO clang complains about missing module
+    # target_compile_definitions(test_package PRIVATE MP_UNITS_MODULES)
     target_link_libraries(test_package PRIVATE mp-units::mp-units)
 endif()