scons: Fix dependencies of generated headers.

scons: Fix dependencies of generated headers.

It appears that scons implicit dependency scanners fail to chain
dependencies of generated headers when these are outside the build tree.

This patch ensures generated source files are _always_ put in the build
tree. I'm not 100% this will fix all depency issues, but from my
experiments it does seem to fix this.

NOTE: For this to be effective it is necessary to clean the source tree
from generated header/source files.

Reviewed-by: Brian Paul <brianp@vmware.com>