make DBUS dependency optional.

It's typically used for Linux Desktop, not OSX and Windows.
The database core implementation based on DBUS was only used with olde KIOSalve which are now removed.
So typically, this code can be removed later, when Internal server will be rewritten without to use DBUS.
Currently, if DBUS is disabled, Internal Server is not available.
CCMAIL: swatilodha27@gmail.com
BUGS:123455
FIXED-IN: 5.0.0