unsetenv: On native Windows, don't modify _environ directly. * m4/setenv.m4 (gl_PREREQ_UNSETENV): Check for _putenv. * lib/unsetenv.c (unsetenv): Add native Windows handling, from lib/putenv.c. * modules/unsetenv (Depends-on): Add free-posix, malloc-posix. * m4/putenv.m4 (gl_FUNC_PUTENV): Use AC_CHECK_DECLS_ONCE. * lib/putenv.c (_unsetenv): Moved to lib/unsetenv.c. (putenv): Invoke unsetenv instead of _unsetenv. * modules/putenv-gnu (Depends-on): Add unsetenv.