Add OS/2-dependent aliases.

Support aliases available on OS/2, especially results of
kLIBC nl_langinfo(CODESET).

* Makefile.devel (all): Depend on lib/aliases_sysos2.h.
(lib/aliases_sysos2.h lib/canonical_sysos2.h
lib/canonical_local_sysos2.h): New rules.
(lib/aliases_os2.h lib/canonical_os2.h): New rules.
(lib/aliases_os2_sysos2.h lib/canonical_os2_sysos2.h): New rules.
(totally-clean): Remove lib/aliases_sysos2.gperf, lib/aliases_os2.h,
lib/aliases_os2_sysos2.h, lib/canonical_sysos2.h, lib/canonical_os2.h,
lib/canonical_os2_sysos2.h, lib/canonical_local_sysos2.h.
* lib/Makefile.in (iconv.lo): Depend on $(srcdir)/aliases_sysos2.h,
$(srcdir)/aliases_os2.h, $(srcdir)/aliases_os2_sysos2.h.
(GENERATED_FILES): Add aliases_sysos2.gperf, aliases_sysos2.h,
aliases_os2.h, aliases_os2_sysos2.h, canonical_sysos2.h,
canonical_os2.h, canonical_os2_sysos2.h, canonical_local_sysos2.h.
* lib/aliases2.h: Include headers for OS/2 aliases.
* lib/encodings.def: Add DEFALIAS statements, conditional on
USE_OS2_ALIASES.
* lib/encodings_aix.def: Add DEFALIAS statements, conditional on
USE_OS2_ALIASES.
* lib/encodings_dos.def: Add DEFALIAS statements, conditional on
USE_OS2_ALIASES.
* lib/genaliases2.c (main): Include encodings_aix.def and
encodings_dos.def for OS/2.
* lib/genflags.c: Define USE_OS2.
* lib/iconv.c: Define USE_OS2 if ENABLE_EXTRA, otherwise define USE_OS2,
USE_AIX, USE_DOS on OS/2.
(all_encodings): Include OS/2-dependent headers on OS/2 aliases.
(aliases2_lookup): Define OS/2-dependent alias lookup table.
(all_canonical): Include OS/2-dependent headers on OS/2 aliases.