Make sure the DTrace header files are built before depend and before
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