Reported by Daniel Richard G. in
<https://lists.gnu.org/archive/html/bug-gnulib/2019-12/msg00172.html>.
* lib/iconv_open-zos.gperf: New file.
* modules/iconv_open (Files): Add iconv_open-zos.gperf.
(Makefile.am): Add rules for generating iconv_open-zos.h from it.
* lib/iconv_open.c (ICONV_FLAVOR_ZOS): New macro.
* m4/iconv_open.m4 (gl_FUNC_ICONV_OPEN): On z/OS, use ICONV_FLAVOR_ZOS.
* doc/posix-functions/iconv_open.texi: Mention z/OS.