Fixed +default+pgsql will not configure with pdo drivers issue

1. Fixed +default+sqlite or +dbs+sqlite will now configure --with-pdo-sqlite correctly.

2. +default or +dbs virtual variants will add combined variants to 'use' array, and support recursive buildVariant .

Ex.
```
phpbrew +sqlite+default+sqlite+sqlite+sqlite+dbs
```
will generate bellow and without duplicate arguments
```
--enable-pdo --with-sqlite3 --with-pdo-sqlite
```

Signed-off-by: Rack Lin <racklin@gmail.com>