dirstamp: switch to a pattern rule

We can leverage $(@D) to generate a single pattern rule for all dirstamp
rules.  This saves many lines in the output -- normally we create 2 rules
(or 6 lines) per subdir, and projects that use subdirs tend to use them
quite a bit.

In the most extreme & unlikely case (1 subdir, no depdir support), the
line count is the same.  In every other case, it's always a win.

Looking at a few real world projects, the line deltas:
* GNU libgloss: +3 -66
* GNU newlib:   +3 -714
* GNU sim:      +3 -138

There shouldn't be any concerns about portability with $(@D) because:
(0) This has been in POSIX (and beyond) for decades,
(1) We only generate this rule iff we know the dirstamp is in a subdir
    (so we'd never have a case where $(@D) would expand to the cwd, and
    that is where a few implementations are known to be buggy),
(2) We already rely on $(@D) in our depdir code, and have since 2014
    (the Automake 1.16 release).

1 files changed, 8 insertions, 4 deletions

diff --git a/bin/automake.in b/bin/automake.in
index 1c13a3187..139d5ad93 100644
--- a/bin/automake.in
+++ b/bin/automake.in
@@ -7893,6 +7893,14 @@ sub require_build_directory
   $directory_map{$directory} = $dirstamp;
   $directory_map{$cdir} = $dirstamp;
 
+  # Generate the pattern rule only once.
+  if (! vardef ('am__dirstamp', TRUE))
+    {
+      $output_rules .= ("%/\$(am__dirstamp):\n"
+			. "\t\@\$(MKDIR_P) \$(\@D)\n"
+			. "\t\@: >>\$\@\n");
+    }
+
   # Set a variable for the dirstamp basename.
   define_pretty_variable ('am__dirstamp', TRUE, INTERNAL,
 			  '$(am__leading_dot)dirstamp');
@@ -7900,10 +7908,6 @@ sub require_build_directory
   # Directory must be removed by 'make distclean'.
   $clean_files{$dirstamp} = DIST_CLEAN;
 
-  $output_rules .= ("$dirstamp:\n"
-		    . "\t\@\$(MKDIR_P) $directory\n"
-		    . "\t\@: >>$dirstamp\n");
-
   return $dirstamp;
 }