setenv: Add missing declaration on OSF/1 5.1. * lib/stdlib.in.h (setenv): Test HAVE_DECL_SETENV, not HAVE_SETENV. * m4/setenv.m4 (gl_FUNC_SETENV_SEPARATE): Test whether setenv is declared. Don't set HAVE_SETENV. * m4/stdlib_h.m4 (gl_STDLIB_H_DEFAULTS): Initialize HAVE_DECL_SETENV, not HAVE_SETENV. * modules/stdlib (Makefile.am): Substitute HAVE_DECL_SETENV, not HAVE_SETENV. * doc/posix-functions/setenv.texi: Mention the OSF/1 5.1 problem.