If we just made it optional then the few people who cared would be
required to set CPPFLAGS and/or LDFLAGS appropriately.
True, but `it's POSIX' and that was used for iconv to make it mandatory.
And optional means the code and the tests have to be that bit more
If this was used in more than one tiny location, I'd find this compelling.
But it's not quite the same as our use of iconv(), which happens in a bunch
of places and can be legitimately be argued as core functionality.