Building Java.inc does not work in PHP7+

[belgattitude]

The buildJavaInc.php is silently failing with with PHP7+ versions.

The Java.inc client is not required to run the php-java-bridge, but the current ant build process still requires it. It's gonna be replaced in the future by the the soluble-japha client and should not be included anymore.

Still for the current version, those fixes are temporarily needed:

• The ant building process should fail with a message for PHP7+ versions.
• Document how to install multiple php versions (see below)
• The ant build.xml contains a php_exec property that you can use to select the required php executable. For example:

  $ ant build -Dphp_exec=/usr/bin/php5.6

---

Tips to install pre-php7 version on Ubuntu:

In many distributions, multiple PHP versions can coexists: on Ubuntu for example, you can install a ppa

sudo add-apt-repository -y ppa:ondrej/php;
sudo apt-get update;

Then install a different php interpreter:

sudo apt-get install php5.6-cli php5.6-mbstring php5.6-zip php5.6-intl php5.6-common php5.6-xml

Once done, run you ant build with

$ ant build -Dphp_exec=/usr/bin/php5.6