Build fails against recent Clang/LLVM

[Alexey Dokuchaev]

As of libc++19[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 breaks GNU APL on the FreeBSD package
build cluster[2].  I've briefly glanced over the problematic code and
it looks like it may need more than simply adjusting the template or
enum types here or there.  Perhaps it is time to adapt to the new
standard before GCC stops accepting current code as well?

./danfe

[1] https://libcxx.llvm.org/ReleaseNotes/19.html#deprecations-and-removals
[2] 
https://pkg-status.freebsd.org/beefy18/data/main-amd64-default/p7c3dbec016b0_s054c5ddf587/logs/gnu-apl-1.8_3.log