[deb] Support for php7 and mariadb

- As discussed on https://sourceforge.net/p/vufind/mailman/message/35002108/
- These changes won't break the previous behaviour, they will just introduce new alternative dependencies based on virtual packages that can either provide php5 or php7 / mysql or mariadb
- Note that explicit apache2 php module dependency is needed because currently the virtual package php5 depends on libapache2-mod-php5 but the virtual package php uses php7-fpm by default, so by making the libapache2-mod-php package need explicit we won't run into cases where the user installed php7-fpm or the virtual package php choose php7-fpm instead of the apache's one.
- Note: Installing vufind without libapache2-mod-php or libapache2-mod-php5 fails with an apache errors as it doesn't recognize vufind.conf, which uses php keywords.
- Please also note that the first package in the precedence order [libapache2-mod-php5] is already a dependency from php5, so this won't break previous functionality.

packages/DEBIAN/control

@@ -3,7 +3,20 @@ Version: 2.5.3
 Section: World Wide Web
 Priority: Optional
 Architecture: all
-Depends: default-jdk, apache2, php5, php5-dev, php-pear, php5-intl, php5-json, php5-ldap, php5-mcrypt, php5-mysql, php5-xsl, php5-gd, mysql-server
+Depends: apache2,
+         default-jdk,
+         libapache2-mod-php5 | libapache2-mod-php,
+         mysql-server | virtual-mysql-server-core,
+         php-pear,
+         php5 | php,
+         php5-dev | php-dev,
+         php5-gd | php-gd,
+         php5-intl | php-intl,
+         php5-json | php-json,
+         php5-ldap | php-ldap,
+         php5-mcrypt | php-mcrypt,
+         php5-mysql | php-mysql,
+         php5-xsl | php-xml
 Maintainer: VuFind Project Administration Team <vufind-admins@lists.sourceforge.net>
 Homepage: http://vufind.org/
 Description: A library resource discovery portal