can't run scons help or clean without installing build-dependencies

[Afif Elghraoui]

Scons checks for dependencies even for just executing the clean or help targets, which is unnecessary and annoying. There is a simple workaround described at

http://www.scons.org/doc/1.2.0/HTML/scons-user/x4105.html

I applied it for 0.8.1 in the Debian package, but my patch doesn't apply to the latest source (mostly because most of the file needs to be reindented).