Add a Run Script phase to your Xcode project and add this script to detect if you use autolayout in your 5.x target

gistfile1.pl

#!/usr/bin/perl
#
# Check if we're running for 5.1 deployment and any .xib view has autolayout on.
# 

use File::Find qw(find);

my $deployment = $ENV{'IPHONEOS_DEPLOYMENT_TARGET'};
my $project_dir = $ENV{'PROJECT_DIR'};
print "Deploy to $deployment\n";
print "Project is $project_dir\n";

# check if deployment target is 6.x
if ($deployment =~ "^6") {
  exit 0;
}

my @xib_files;

# find all xib files
find ({ wanted => sub { /^.*\.xib\z/s && push @xib_files, $_ } , no_chdir => 1 }, $project_dir);

$has_error = 0;

for my $file (@xib_files) {
	local $/=undef;
	open FILE, $file || die "Cant open $file";
	$data = <FILE>;
	close FILE;
	(($has_error = 1) && print "error: $file has autolayout\n") if ($data =~ /UseAutolayout\">YES/m);
}

if ($has_error) {
	exit 1;
}

exit 0;