unsetenv: Add missing declaration on OSF/1 5.1.

unsetenv: Add missing declaration on OSF/1 5.1.

* lib/stdlib.in.h (setenv): Test HAVE_DECL_UNSETENV, not HAVE_UNSETENV.
* m4/setenv.m4 (gl_FUNC_UNSETENV): Test whether unsetenv is declared.
Don't set HAVE_UNSETENV. In the test program, set _BSD.
* m4/stdlib_h.m4 (gl_STDLIB_H_DEFAULTS): Initialize HAVE_DECL_UNSETENV,
not HAVE_UNSETENV.
* modules/stdlib (Makefile.am): Substitute HAVE_DECL_UNSETENV, not
HAVE_UNSETENV.
* doc/posix-functions/unsetenv.texi: Mention the OSF/1 5.1 problem.