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)

.travis.yml

@@ -0,0 +1,28 @@
+dist: bionic
+
+sudo: false
+
+language: php
+
+php:
+  - 7.1
+  - 7.2
+  - 7.3
+  - 7.4
+
+before_install:
+  - sudo sh -c 'echo "deb https://repo.delellis.com.ar bionic bionic" > /etc/apt/sources.list.d/20-pdlib.list'
+  - wget -qO - https://repo.delellis.com.ar/repo.gpg.key | sudo apt-key add -
+  - sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 762E3157
+  - sudo apt update
+  - sudo apt-get install libdlib-dev
+  - phpize
+  - ./configure --enable-debug
+  - make
+  - sudo make install
+  - echo "extension=pdlib.so" >> `php --ini | grep "Loaded Configuration" | sed -e "s|.*:\s*||"`
+
+script:
+  - export TEST_PHP_EXECUTABLE=`which php`
+  - export NO_INTERACTION=1
+  - php run-tests.php -q --show-diff