Add native vector_lenght() function which calculates the euclidean distance 80%

faster. See https://github.com/matiasdelellis/pdlib/pull/2.

Increase the version to 1.0.2 since this is a required function for our
application and we must control it.

Add more compilation info in php-info(), and print a final message with the
build options when configure.

Fix that when dlib is not installed, the configuration returns that it cannot
find pkgconfig. See https://github.com/matiasdelellis/facerecognition/issues/261

Also implement the test on travis. See for example:
(https://travis-ci.com/github/matiasdelellis/pdlib/builds/161854044)

tests/vector_length.phpt

@@ -0,0 +1,20 @@
+--TEST--
+Basic tests for dlib_vector_length
+--SKIPIF--
+<?php if (!extension_loaded("pdlib")) print "skip"; ?>
+--FILE--
+<?php
+$vectorAL = [0.0, 0.0];
+$vectorAR = [0.0, 1.0];
+var_dump(dlib_vector_length($vectorAL, $vectorAR));
+$vectorBL = [0.0, 0.0, -1.0];
+$vectorBR = [0.0, 0.0, 1.0];
+var_dump(dlib_vector_length($vectorBL, $vectorBR));
+$vectorCL = [0.0, 2.5, 1.0];
+$vectorCR = [0.0, 1.0, 1.0];
+var_dump(dlib_vector_length($vectorCL, $vectorCR));
+?>
+--EXPECT--
+float(1)
+float(2)
+float(1.5)

tests/version.phpt

@@ -0,0 +1,10 @@
+--TEST--
+Just test php extension version
+--SKIPIF--
+<?php if (!extension_loaded("pdlib")) print "skip"; ?>
+--FILE--
+<?php
+var_dump(phpversion('pdlib'));
+?>
+--EXPECT--
+string(5) "1.0.2"