devel/poco: fix build with libc++ 19
As noted in the libc++ 19 release notes [1], std::char_traits<> is now
only provided for char, char8_t, char16_t, char32_t and wchar_t, and any
instantiation for other types will fail.
This causes devel/poco to fail to compile with clang 19 and libc++ 19,
resulting in errors similar to:
/usr/include/c++/v1/string:820:42: error: implicit instantiation of undefined template 'std::char_traits<unsigned char>'
820 | static_assert(is_same<_CharT, typename traits_type::char_type>::value,
| ^
/usr/include/c++/v1/__type_traits/is_constructible.h:42:62: note: in instantiation of template class 'std::basic_string<unsigned char>' requested here
42 | : public integral_constant<bool, __is_constructible(_Tp, __add_rvalue_reference_t<_Tp>)> {};
| ^
/usr/include/c++/v1/__type_traits/is_swappable.h:43:39: note: in instantiation of template class 'std::is_move_constructible<Poco::Data::LOB<unsigned char>>' requested here
43 | using __swap_result_t = __enable_if_t<is_move_constructible<_Tp>::value && is_move_assignable<_Tp>::value>;
| ^
/usr/include/c++/v1/__type_traits/is_swappable.h:50:60: note: in instantiation of template type alias '__swap_result_t' requested here
50 | inline _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX20 __swap_result_t<_Tp> swap(_Tp& __x, _Tp& __y)
| ^
/wrkdirs/usr/ports/devel/poco/work/poco-1.12.4-all/Data/include/Poco/Data/LOB.h:243:14: note: while substituting explicitly-specified template arguments into function template 'swap'
243 | inline void swap<Poco::Data::BLOB>(Poco::Data::BLOB& b1, Poco::Data::BLOB& b2) noexcept
| ^In this case it is enough to only enable the LOB constructor from
std::basic_string when the type is plain char.
[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals
PR: 281553
MFH: 2024Q3