iconv-h: Fix test-iconv-h-c++ failure on Solaris 11 2010-11.

iconv-h: Fix test-iconv-h-c++ failure on Solaris 11 2010-11.

* lib/iconv.in.h (iconv_open, iconv, iconv_close): Define only if the
'iconv' module is present.
(ICONV_CONST): New macro.
* m4/iconv_h.m4 (gl_ICONV_H_DEFAULTS): Initialize GNULIB_ICONV and
ICONV_CONST.
* m4/iconv.m4 (AM_ICONV): If the gnulib module 'iconv-h' is present,
set ICONV_CONST.
* m4/iconv_open.m4 (gl_FUNC_ICONV_OPEN_UTF): Don't set ICONV_CONST
here.
* modules/iconv (configure.ac): Invoke gl_ICONV_MODULE_INDICATOR.
* modules/iconv-h (Makefile.am): Substitute GNULIB_ICONV.
* tests/test-iconv-h.c (ICONV_CONST): Don't define here.
* tests/test-iconv-h-c++.cc (ICONV_CONST): Don't define here.
(iconv_open, iconv, iconv_close): Test only if the 'iconv' module is
present.