ooinstall: use DESTDIR also for the Cwd::realpath call

this solution is not perfect because Cwd::realpath($ENV{DESTDIR})
need not be part of Cwd::realpath($ENV{DESTDIR} . $arg). Let's
solve this when it really cause troubles.

solenv/bin/ooinstall

@@ -30,7 +30,9 @@ for $arg (@ARGV) {
     } else {
 	# Cwd::realpath does not work if the path does not exist
 	mkpath($ENV{DESTDIR} . $arg) unless -d $ENV{DESTDIR} . $arg;
-	$path = Cwd::realpath( $arg );
+	$path = Cwd::realpath( $ENV{DESTDIR} . $arg );
+	my $destdir = Cwd::realpath( $ENV{DESTDIR} );
+	$path =~ s|$destdir||;
     }
 }