Show built-in features in --version output

Example:
```
Aug 24 21:54:12 PowerDNS Authoritative Server git-auth-3.3-513-gcd91689+ (ch@nq.home.zeha.at) (C) 2001-2013 PowerDNS.COM BV
Aug 24 21:54:12 Using 64-bits mode. Built on 20130824215356 by ch@nq.home.zeha.at, gcc 4.8.1.
Aug 24 21:54:12 PowerDNS comes with ABSOLUTELY NO WARRANTY. This is free software, and you are welcome to redistribute it according to the terms of the GPL version 2.
Aug 24 21:54:12 Features: libdl lua 
Aug 24 21:54:12 Built-in modules: gsqlite3 ldap opendbx
```

Partially addresses #958.