iconv_open, iconv_open-utf: Respect rules for use of AC_LIBOBJ.

iconv_open, iconv_open-utf: Respect rules for use of AC_LIBOBJ.

If module 'iconv_open' is among the main modules and module
'iconv_open-utf' is among the tests dependencies, then
REPLACE_ICONV_UTF will be defined to 1, hence iconv_open() in lib may
return the special iconv_t values. Therefore iconv() and iconv_close()
must support these special iconv_t values, already in lib, not only in
tests.
* m4/iconv_open-utf.m4: New file, extracted from m4/iconv_open.m4.
* m4/iconv_open.m4 (gl_FUNC_ICONV_OPEN): Invoke
gl_FUNC_ICONV_OPEN_UTF_SUPPORT if present.
(gl_FUNC_ICONV_OPEN_UTF): Remove macro.
* modules/iconv_open (Files): Add lib/iconv.c, lib/iconv_close.c.
(Depends-on): Add the dependencies of iconv_open-utf.
* modules/iconv_open-utf (Files): Add m4/iconv_open-utf.m4. Remove
m4/iconv_open.m4, lib/iconv.c, lib/iconv_close.c.
(Depends-on): Remove modules needed by lib/iconv.c, lib/iconv_close.c.