Eliminate use of BUILT_SOURCES from Gnulib.

As with recursive make, BUILT_SOURCES is harmful because of inadequate
dependency information.  We can achieve a similar effect with by using
order-only dependencies:  These force the headers to be up-to-date
before anything that might require them is built: but rebuilds are only
triggered based on the accurate dependency information generated by the
normal mechanisms.

Unfortunately, order-only dependencies are a GNU make feature, so we use
a hack which should fall back to ordinary dependencies on other make
implementations.  The worst effect of using ordinary dependencies will
be that files might be needlessly rebuilt when a header changes.

diff --git a/scripts/fix-gnulib.pl b/scripts/fix-gnulib.pl
index 6663e6e4b49c333eb95b53fcb10b0657f6d7417c..b93cf22b44dc5fe042057ca4b9ec462c7541c106 100755 (executable)
--- a/scripts/fix-gnulib.pl
+++ b/scripts/fix-gnulib.pl
@@ -87,7 +87,14 @@ while (<STDIN>) {
        next if (/^#/);
 
        if (!$printed_header) {
-               print "# Postprocessed by ", basename($0), "\n";
+               print "# Postprocessed by ", basename($0), "\n\n";
+               print <<'EOF';
+# This trick should define gnulib_orderonly to | iff we're using GNU make.
+gnulib_have_orderonly = $(findstring order-only,$(.FEATURES))
+gnulib_orderonly = $(gnulib_have_orderonly:order-only=|)
+gnulib_headers = $(gnulib_orderonly)
+EOF
+
                $printed_header = 1;
                drop;
        }
@@ -122,6 +129,11 @@ while (<STDIN>) {
                }
        }
 
+       # BUILT_SOURCES has similar problems to recursive make: inadequate
+       # dependencies lead to incorrect builds.  Collect them into an
+       # ordinary variable so we can deal with them later.
+       s/BUILT_SOURCES/gnulib_headers/;
+
        # Targets are similar to variables: the target and its dependencies
        # need to be mangled.
        if (/:/) {
@@ -133,6 +145,8 @@ while (<STDIN>) {
        s:\$\(builddir\):\$\(top_builddir\)/lib:g;
 } continue { print };
 
+print "\$(libgnu_la_OBJECTS): \$(gnulib_headers)\n";
+
 # Some filenames are AC_SUBSTed by the Gnulib macros, and thus we need to
 # prepend lib/ if and only if they're not empty.  Unfortunately, make is not
 # powerful to do this, so we need to put this transformation into configure