m4/getcwd-path-max.m4 hangs with busybox + btrfs

[NightStrike <nightstrike@gmail.com>]

See https://bugs.gentoo.org/447970 for extra details.  While this
gentoo report has a workaround posted just a few months ago, it seems
that the test itself is faulty, as a failure mode should not be to
hang the entire computer.

When running this configure test on my particular system, a NAS
running busybox and btrfs, it hung for at least 12 hours before I
decided to power cycle it.  While hung, the system is mostly
unresponsive.  Pings work, for instance, but no new processes can
start, and the RCU kernel thread spins its core at 100%.  The test
states that it is needed for old glibc and kernel versions.  My NAS is
using glibc 2.27 and kernel 5.13, so presumably it is no longer a
useful test.  Can this be modified to be less destructive?

I mention btrfs, because that gentoo link highlights that the test is
particularly troublesome on that file system.

I mention busybox, because when I inspect the leftover directory
structure after rebooting, I find that the busybox shell cannot handle
the path length, with tab completion eventually failing.

In my case, I hit this while configuring m4 version 1.4.19, which uses
the latest (serial 25) version of this file.