auto-detection of configure changes

[tridge at samba.org]

Hi Metze,

This is in regard to the waf TODO item of auto-detecting changes to
the build scripts that require configure to be re-run.

I've added this as an option, but have not enabled it by default. To
enable it use "configure --enable-auto-reconfigure".

The reason it is off by default is that it interacts badly with the
configure -C option, and ends up inheriting the private WAFCACHE
setting used in the implementation of -C into the main build.

We would also need to split out the build portions of scripts like
lib/talloc/wscript into lib/talloc/wscript_build, as otherwise you'll
get a reconfigure on any build rule changes in those scripts as well.

I'd prefer to leave this off by default for now, and consider
enabling it by default once these problems are fixed and we have been
using the waf build for a bit longer. I don't want to destabilise
things just when I am proposing that waf become the default build
system for Samba4.

Cheers, Tridge