Buildsystem diff

Hi,

This patch makes the garble plugin a default plugin in all builds but
Windows. By default the garble plugin does NOTHING:
- no initialization code is executed
- no menu items are added
- not a single feature is added anywhere

It could however be very handy to have this plugin available by default,
so anyone who has bugs in a proprietary document can be instructed to
garble his documents text/image content, so they can be safely uploaded
to Abiword's bugzilla.

Not committing this myself for obvious reasons.

Cheers!
Marc