make_dlls: Generate the .gitignore file.

dlls/.gitignore

+# Automatically generated by make_dlls; DO NOT EDIT!!
 /Makedll.rules
 /Makeimplib.rules
 /Maketest.rules

dlls/make_dlls

@@ -351,3 +351,31 @@ EOF
 close NEWMAKE;
 rename "Makefile.in.new", "Makefile.in";
 printf "Successfully updated Makefile.in\n";
+
+################################################################
+# .gitignore file
+
+open GITIGNORE, ">.gitignore.new" or die "cannot create .gitignore.new";
+print GITIGNORE "# Automatically generated by make_dlls; DO NOT EDIT!!\n";
+
+my @ignores =
+(
+ "/Makedll.rules",
+ "/Makeimplib.rules",
+ "/Maketest.rules",
+);
+
+foreach my $mod (sort @lib_symlinks)
+{
+    push @ignores, "/$importlibs{$mod}.def";
+}
+foreach my $mod (sort keys %directories)
+{
+    next unless defined $altnames{$mod};
+    push @ignores, map { "/" . $_ . "16"; } @{$altnames{$mod}};
+}
+print GITIGNORE join("\n", sort @ignores) . "\n";
+
+close GITIGNORE;
+rename ".gitignore.new", ".gitignore";
+printf "Successfully updated .gitignore\n";