Replaced unsigned int unpack template to used signed (per puzzle.h).

RT-113716: signature_as_char_string was using the unsigned unpack
template to convert the signature to a character representation; the
reporter pointed out that the signature is actually a signed character
and that the resulting string representation was not correct.

https://rt.cpan.org/Ticket/Display.html?id=113716

The suggested fix was implemented; the unpack now uses the signed char
template (c); this results in an array of values that range -2..2; the
arrays is then iterated and 2 is added to each value; this shifts the
range to 0..4 (removes the minus sign), and therefore produces stringified
signatures that are of a consistent length from image to image. Now
this makes character string cvecs (and ngrams) usable for image indexing.

Minor unrelated change - removed the check for puzzle.h. In theory it is
nice to have; but it was causing issues on my dev machine, FreeBSD 10.3.

Makefile.PL

@@ -1,13 +1,6 @@
 use 5.014004;
 use ExtUtils::MakeMaker;
 
-# Check that the local system has libpuzzle installed using
-# Devel::CheckLib.
-
-use lib 'inc';
-use Devel::CheckLib;
-check_lib_or_exit (lib => 'puzzle', header => 'puzzle.h');
-
 # See lib/ExtUtils/MakeMaker.pm for details of how to influence
 # the contents of the Makefile that is written.
 my $repo = 'https://github.com/estrabd/Image-Libpuzzle';

lib/Image/Libpuzzle.pm

@@ -17,13 +17,15 @@ our $PUZZLE_CVEC_SIMILARITY_HIGH_THRESHOLD  = Image::Libpuzzle->PUZZLE_CVEC_SIMI
 our $PUZZLE_CVEC_SIMILARITY_LOW_THRESHOLD   = Image::Libpuzzle->PUZZLE_CVEC_SIMILARITY_LOW_THRESHOLD;
 our $PUZZLE_CVEC_SIMILARITY_LOWER_THRESHOLD = Image::Libpuzzle->PUZZLE_CVEC_SIMILARITY_LOWER_THRESHOLD;
 
-# uses unpack as bin to char and $self accessor to get signature directly from the internal cvec
+# uses unpack as bin to insigned char, then add 2 to remove negative number; provides range of 0..4;
+#  and $self accessor to get signature directly from the internal cvec
 sub signature_as_char_string {
   my $self = shift;
-  my @sig  = unpack("C*", $self->get_signature());
+  # take as 
+  my @sig  = unpack("c*", $self->get_signature());
   my $sig  = q{};
   foreach my $i (@sig) {
-    $sig .= sprintf("%02d", $i);
+    $sig .= sprintf("%02d", $i+2);
   }
   return $sig;
 }