getlogin: Port to newer mingw. * m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Initialize HAVE_DECL_GETLOGIN. * m4/getlogin.m4 (gl_FUNC_GETLOGIN): Set HAVE_DECL_GETLOGIN. * modules/unistd (Makefile.am): Substibute HAVE_DECL_GETLOGIN, not HAVE_GETLOGIN. * lib/unistd.in.h (getlogin): Test HAVE_DECL_GETLOGIN, not HAVE_GETLOGIN. * doc/posix-functions/getlogin.texi: Mention the issue. * tests/test-getlogin.c: Don't include <pwd.h> on native Windows.