clang: rm gcc flag

cmake/BornAgain/Linux.cmake

@@ -18,6 +18,8 @@ set(CMAKE_C_FLAGS_RELEASE          "-O3")
 set(CMAKE_C_FLAGS_DEBUG            "-g")
 
 string(APPEND CMAKE_CXX_FLAGS " -I${CMAKE_SOURCE_DIR}/Wrap -pipe ${BIT_ENVIRONMENT} -Wall -W -Woverloaded-virtual -Wno-unknown-pragmas -fPIC")
-string(APPEND CMAKE_CXX_FLAGS " -Wno-restrict") # under g++ -std=c++20, warnings from <string>
+if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    string(APPEND CMAKE_CXX_FLAGS " -Wno-restrict") # under g++ -std=c++20, warnings from <string>
+endif()
 string(APPEND CMAKE_C_FLAGS " -pipe ${BIT_ENVIRONMENT} -Wall -W -fPIC")
 set(CMAKE_SHARED_LINKER_FLAGS "-Wl,--no-undefined")