the build starts.
This adds a new variable DHDRS that contains a list of all DTrace
header files. Then, we use the beforedepend hook to make sure the
heaeder files are built.
Introduce a beforebuild dependency (from projects/bmake) based on
feedback from Simon J. Gerraty. This lets us generate the header
files without running make depend.
Reviewed by: sjg, imp
MFC after: 3 days