Work around unportability of nl_langinfo(CODESET).

Work around unportability of nl_langinfo(CODESET).

* m4/glibc21.m4: New file, from gnulib.
* Makefile.sub (M4MACROS): Add m4/glibc21.m4.
* configure.ac: Also test for stddef.h, and invoke jm_GLIBC21.
* Makefile.in (HOST, GLIBC21): New variables.
(MDEFINES): Pass them to subdirectories.
* src/include/relocate.h (relocatep): Define with C linkage.
* src/include/localcharset.h: New file, from gnulib.
* src/libs/libgroff/localcharset.c: New file, from gnulib with a
modification for relocate().
* src/libs/libgroff/config.charset: New file, from gnulib.
* src/libs/libgroff/ref-add.sin: New file, from gnulib.
* src/libs/libgroff/ref-del.sin: New file, from gnulib.
* src/libs/libgroff/Makefile.sub (EXTRA_CFLAGS): Also define LIBDIR.
(OBJS): Add localcharset.o.
(CSRCS): Add localcharset.c.
(all): Add dependencies to charset.alias, ref-add.sed, ref-del.sed.
(charset.alias): New rule.
(PACKAGE): New variable.
(ref-add.sed, ref-del.sed): New rules.
(MKINSTALLDIRS): New variable.
(install_data): Depend on install_charset_data.
(install_charset_data): New rule for creating or updating
charset.alias.
(uninstall_sub): Depend on uninstall_charset_data.
(uninstall_charset_data): New rule for uninstalling or updating
charset.alias.
* src/preproc/preconv/preconv.cpp: Include localcharset.h instead
of <langinfo.h>.
(main): Initialize default_encoding from locale_charset() instead of
nl_langinfo(CODESET).

* aclocal.m4, configure, src/include/config.hin: Regenerated.