infrastructure for autoreconf

[Tobias Nygren <tnn%NetBSD.org@localhost>]

Hi all,

We have quite a few packages that TOOL_DEPEND on autoconf/automake and
override the pre-configure target with a rules that does
"cd ${WRKSRC} && autoreconf".

This pattern is cargo culted all over the tree and I think we can do
better, at least for the straight forward packages. Either we could
extend GNU_CONFIGURE to accept other values than yes/no, for example:

GNU_CONFIGURE=	regenerate
or maybe
GNU_CONFIGURE=	autoreconf

Or we could introduce a new companion variable.

GNU_CONFIGURE=	yes
USE_AUTORECONF=	yes

I prefer the first option for being the most compact.
Let's bikeshed for a while about the best syntax and add it to mk/?

-Tobias