build: Update gettext infrastructure files

* build-aux/config.rpath:  Update from gettext 0.26.
* m4/host-cpu-c-abi.m4: New file, from gettext-0.26.
* m4/iconv.m4: Upgrade to gettext 0.26.
* m4/lib-ld.m4: New file, from gettext-0.26.
* m4/lib-link.m4: New file, from gettext-0.26.
* m4/lib-prefix.m4: New file, from gettext-0.26.
* m4/Makefile.am (EXTRA_DIST): Add the new files.

With gettext 0.26 most of gettext's m4 files are no longer installed in
/usr/share/aclocal. This caused AM_ICONV to fail because several macros
were "m4_require'd but not m4_defun'd". I ran gettextize to update/copy
the necessary files and I removed everything that we don't need because
pinentry doesn't use gettext directly.