graphics/povray37: 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 graphics/povray37 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 short>'
  820 |   static_assert(is_same<_CharT, typename traits_type::char_type>::value,
      |                                          ^
./base/configbase.h:159:19: note: in instantiation of template class 'std::basic_string<unsigned short>' requested here
  159 | inline UCS2String ASCIItoUCS2String(const char *s)
      |                   ^
/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 POVMSUCS2 as char16_t instead of
unsigned short, UCS2 as char16_t instead of unsigned short, and
UCS4 as char32_t instead of unsigned int in the passed-in
CPPFLAGS variable.

Also, patch up vfe.cpp and vfeconf.h to use char16_t and
u16string instead of the non-standard types.

[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals

PR: 282645
Approved by: bsd@philippost.de (maintainer)
MFH: 2024Q4