Fix timing bug on high-speed builds

Problem reported by Bogdan via Jacob Bachmeyer in:
https://lists.gnu.org/r/autoconf/2023-04/msg00002.html
* bin/autom4te.in: If a file timestamp equals a dependency’s
timestamp, consider the file to be out of date.  Although this may
result in extra work, it fixes some rare timing bugs.

bin/autom4te.in

@@ -910,10 +910,8 @@ sub up_to_date ($)
   return 0
     if ! -f $tfile || ! -f $ofile;
 
-  # The youngest of the cache files must be older than the oldest of
+  # The younger of the cache files must be older than the oldest of
   # the dependencies.
-  # FIXME: These timestamps have only 1-second resolution.
-  # Time::HiRes fixes this, but assumes Perl 5.8 or later.
   my $tmtime = mtime ($tfile);
   my $omtime = mtime ($ofile);
   my ($file, $mtime) = ($tmtime < $omtime
@@ -926,7 +924,7 @@ sub up_to_date ($)
   # We depend at least upon the arguments.
   foreach my $dep (@ARGV)
     {
-      if ($mtime < mtime ($dep))
+      if ($mtime <= mtime ($dep))
 	{
 	  verb "up_to_date ($file): outdated: $dep";
 	  return 0;
@@ -949,7 +947,7 @@ sub up_to_date ($)
       # timestamp of that missing file was newer).
       return 0
 	if ! $dep;
-      if ($mtime < mtime ($dep))
+      if ($mtime <= mtime ($dep))
 	{
 	  verb "up_to_date ($file): outdated: $dep";
 	  return 0;
@@ -1038,7 +1036,7 @@ $icache_file = new Autom4te::XFile $icache, O_RDWR|O_CREAT;
 $icache_file->lock (LOCK_EX)
   if ($flock_implemented eq "yes");
 
-# Read the cache index if available and older than autom4te itself.
+# Read the cache index if available and younger than autom4te itself.
 # If autom4te is younger, then some structures such as C4che might
 # have changed, which would corrupt its processing.
 Autom4te::C4che->load ($icache_file)
@@ -1105,7 +1103,7 @@ else
     # Actual M4 expansion, if the user wants it, or if $output is old
     # (STDOUT is pretty old).
     handle_output ($req, $output)
-      if $force || mtime ($output) < mtime ($ocache . $req->id);
+      if $force || mtime ($output) <= mtime ($ocache . $req->id);
   }
 
 # If we ran up to here, the cache is valid.