build: MP_UNITS_BUILD_IMPORT_STD CMake option removed

src/CMakeLists.txt

@@ -38,11 +38,9 @@ check_libcxx_in_use(${projectPrefix}LIBCXX)
 # project build options
 option(${projectPrefix}BUILD_AS_SYSTEM_HEADERS "Export library as system headers" OFF)
 option(${projectPrefix}BUILD_CXX_MODULES "Add C++ modules to the list of default targets" OFF)
-option(${projectPrefix}BUILD_IMPORT_STD "Enable `import std;` usage" OFF)
 
 message(STATUS "${projectPrefix}BUILD_AS_SYSTEM_HEADERS: ${${projectPrefix}BUILD_AS_SYSTEM_HEADERS}")
 message(STATUS "${projectPrefix}BUILD_CXX_MODULES: ${${projectPrefix}BUILD_CXX_MODULES}")
-message(STATUS "${projectPrefix}BUILD_IMPORT_STD: ${${projectPrefix}BUILD_IMPORT_STD}")
 
 if(${projectPrefix}BUILD_AS_SYSTEM_HEADERS)
     set(${projectPrefix}_AS_SYSTEM SYSTEM)
@@ -126,7 +124,7 @@ else()
     set(${projectPrefix}TARGET_SCOPE "INTERFACE")
 endif()
 
-if(${projectPrefix}BUILD_IMPORT_STD)
+if(CMAKE_CXX_MODULE_STD)
     if(CMAKE_VERSION VERSION_LESS "3.30")
         message(FATAL_ERROR "CMake versions before 3.30 do not support `import std;` properly")
     endif()

src/core/CMakeLists.txt

@@ -125,7 +125,7 @@ if(${projectPrefix}BUILD_CXX_MODULES)
     endif()
 endif()
 
-if(${projectPrefix}BUILD_IMPORT_STD)
+if(CMAKE_CXX_MODULE_STD)
     target_compile_definitions(mp-units-core ${${projectPrefix}TARGET_SCOPE} ${projectPrefix}IMPORT_STD)
     # https://github.com/llvm/llvm-project/issues/75057
     if(CMAKE_CXX_COMPILER_ID MATCHES ".*Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19)