dirstamp: use append too instead of truncate

We changed the depfiles logic to use >> (append) instead of > (truncate)
due to it being slightly faster & nicer to the disk.  Do the same with
the dirstamp files as we only need the files to exist -- we don't care
about their content, and we never put anything in them ourselves.  If
someone else were to, we clean them up normally with `make clean`.

Simple test case on my Linux 6.1 w/ext4 on SSD:

@: > foo.txt
for (i = 0; i < 1000000; ++i) close(open("foo.txt", O_WRONLY|O_CREAT|O_TRUNC, 0666));
-> 769 msec

@: >>foo.txt
for (i = 0; i < 1000000; ++i) close(open("foo.txt", O_WRONLY|O_CREAT|O_APPEND, 0666));
-> 2 sec

1 files changed, 1 insertions, 1 deletions

diff --git a/bin/automake.in b/bin/automake.in
index 306913279..1c13a3187 100644
--- a/bin/automake.in
+++ b/bin/automake.in
@@ -7902,7 +7902,7 @@ sub require_build_directory
 
   $output_rules .= ("$dirstamp:\n"
 		    . "\t\@\$(MKDIR_P) $directory\n"
-		    . "\t\@: > $dirstamp\n");
+		    . "\t\@: >>$dirstamp\n");
 
   return $dirstamp;
 }