Reported compiler error on OSX when `BUILD_STATIC` is `ON`

[charlietsao]

Error as follows when building rttr_core_lib:

In file included from /Users/hell//rttr/src/rttr/constructor.cpp:28:
In file included from /Users/hell//rttr/src/rttr/constructor.h:35:
In file included from /Users/hell//rttr/src/rttr/array_range.h:338:
In file included from /Users/hell//rttr/src/rttr/detail/impl/array_range_impl.h:33:
In file included from /Users/hell//rttr/src/rttr/type.h:34:
In file included from /Users/hell//rttr/src/rttr/filter_item.h:32:
/Users/hell//rttr/src/rttr/enum_flags.h:277:67: error: unused parameter 'v' [-Werror,-Wunused-parameter]
        RTTR_CONSTEXPR RTTR_INLINE explicit invalid_enum_flag(int v){}
                                                                  ^
In file included from /Users/hell//rttr/src/rttr/constructor.cpp:28:
In file included from /Users/hell//rttr/src/rttr/constructor.h:35:
In file included from /Users/hell//rttr/src/rttr/array_range.h:338:
In file included from /Users/hell//rttr/src/rttr/detail/impl/array_range_impl.h:33:
In file included from /Users/hell//rttr/src/rttr/type.h:1234:
In file included from /Users/hell//rttr/src/rttr/detail/type/type_impl.h:36:
In file included from /Users/hell//rttr/src/rttr/detail/type/get_create_variant_func.h:32:
In file included from /Users/hell//rttr/src/rttr/variant.h:1157:
In file included from /Users/hell//rttr/src/rttr/detail/variant/variant_impl.h:36:
In file included from /Users/hell//rttr/src/rttr/detail/variant/variant_data_policy.h:34:
In file included from /Users/hell//rttr/src/rttr/detail/variant_associative_view/variant_associative_view_creator.h:34:
In file included from /Users/hell//rttr/src/rttr/associative_mapper.h:363:
/Users/hell//rttr/src/rttr/detail/impl/associative_mapper_impl.h:85:36: error: unused parameter 'itr' [-Werror,-Wunused-parameter]
    get_value(const iterator_data& itr)
                                   ^
/Users/hell//rttr/src/rttr/detail/impl/associative_mapper_impl.h:127:29: error: unused parameter 'container' [-Werror,-Wunused-parameter]
    static void clear(void* container)
                            ^
/Users/hell//rttr/src/rttr/detail/impl/associative_mapper_impl.h:166:36: error: unused parameter 'container' [-Werror,-Wunused-parameter]
    static std::size_t erase(void* container, argument& key)
                                   ^
/Users/hell//rttr/src/rttr/detail/impl/associative_mapper_impl.h:166:57: error: unused parameter 'key' [-Werror,-Wunused-parameter]
    static std::size_t erase(void* container, argument& key)
                                                        ^
/Users/hell//rttr/src/rttr/detail/impl/associative_mapper_impl.h:190:55: error: unused parameter 'key' [-Werror,-Wunused-parameter]
    static bool insert_key(void* container, argument& key, iterator_data& itr)
                                                      ^
/Users/hell//rttr/src/rttr/detail/impl/associative_mapper_impl.h:216:61: error: unused parameter 'key' [-Werror,-Wunused-parameter]
    static bool insert_key_value(void* container, argument& key, argument& value, iterator_data& itr)
                                                            ^
/Users/hell//rttr/src/rttr/detail/impl/associative_mapper_impl.h:216:76: error: unused parameter 'value' [-Werror,-Wunused-parameter]
    static bool insert_key_value(void* container, argument& key, argument& value, iterator_data& itr)
                                                                           ^
/Users/hell//rttr/src/rttr/detail/impl/associative_mapper_impl.h:310:83: error: unused parameter 'key' [-Werror,-Wunused-parameter]
    static std::pair<itr_t, bool> insert_key(container_t& container, const key_t& key)
                                                                                  ^
/Users/hell//rttr/src/rttr/detail/impl/associative_mapper_impl.h:459:39: error: unused parameter 'itr_tgt' [-Werror,-Wunused-parameter]
    static void create(iterator_data& itr_tgt, const iterator_data& src)
                                      ^
/Users/hell//rttr/src/rttr/detail/impl/associative_mapper_impl.h:459:69: error: unused parameter 'src' [-Werror,-Wunused-parameter]
    static void create(iterator_data& itr_tgt, const iterator_data& src)

-Wno-unused-parameter should be added to the function set_compiler_warnings target in utility.cmake.