m4: speed up filesystem modification checks

m4: speed up filesystem modification checks

The current code sleeps at least 1 second to make sure the generated
files are strictly newer than the source files.  It does this for a
few reasons: POSIX only guarantees that `sleep` accept integers, and
filesystems have a history (c.f. Windows) of bad timestamp resolution.

For the first part, we can easily probe sleep to see if it accepts a
decimal number with a fractional part -- just run `sleep 0.001`.

For the second part, we can create two files and then run sleep in a
loop to see when one is considered newer than the other.

For many projects, this 1 second delay is largely amortized by the
rest of the configure script.  Autoconf lends itself to being both
large & slow.  But in projects with many smallish configure scripts
with many cached vars, the time to rerun is dominated by this single
sleep call.  For example, building libgloss against a compiler with
many (60+) multilib configurations, we see:
[Using sleep 1]
$ time ./config.status
real    2m28.164s
user    0m33.651s
sys     0m9.083s
[Using sleep 0.1]
$ time ./config.status
real    0m39.569s
user    0m33.517s
sys     0m8.969s

And in case anyone wonders, going below 0.1s doesn't seem to make a
statistically significant difference, at least in this configuration.
It appears to be within "noise" limits.
[Using sleep 0.001]
$ time ./config.status
real    0m39.760s
user    0m33.342s
sys     0m9.080s

* NEWS: Mention updated timestamp checking.
* m4/sanity.m4: Determine whether `sleep` accepts fractional seconds.
Determine (roughly) the filesystem timestamp resolution.  Use this to
sleep less when waiting for generated file timestamps to update.