editors/abiword: 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 editors/abiword 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 int>'
820 | static_assert(is_same<_CharT, typename traits_type::char_type>::value,
| ^
ut_string_class.cpp:1275:41: note: in instantiation of template class 'std::basic_string<unsigned int>' requested here
1275 | : pimpl(new UT_StringImpl<UT_UCS4Char>(*rhs.pimpl))
| ^
/usr/include/c++/v1/__fwd/string.h:23:29: note: template is declared here
23 | struct _LIBCPP_TEMPLATE_VIS char_traits;
| ^This can be fixed by defining UT_UCS4Char as char32_t instead of
gunichar, and patching up a few places where those types are mixed
indiscriminately.
[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals
PR: 285657
Approved by: gnome (fluffy)
MFH: 2025Q1
(cherry picked from commit e6daa211c609bde6ed3fe00fd5ce675714371805)