perl: install private Error.pm if the site version is older than our own

bdash (on IRC) had a problem with Git.pm (via git-svn) when his
site installation of Error.pm was older than the version we
package.
Signed-off-by: NEric Wong <normalperson@yhbt.net>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

perl/Makefile.PL

@@ -13,7 +13,7 @@ my %pm = ('Git.pm' => '$(INST_LIBDIR)/Git.pm');
 # We come with our own bundled Error.pm. It's not in the set of default
 # Perl modules so install it if it's not available on the system yet.
 eval { require Error };
-if ($@) {
+if ($@ || $Error::VERSION < 0.15009) {
 	$pm{'private-Error.pm'} = '$(INST_LIBDIR)/Error.pm';
 }