Unified build - fix make depend

There was a catch 22, where 'make depend' directly after configuring
in an otherwise pristine build tree would fail because buildinf.h
didn't exist yet.

This change has the depend building targets depend on the same other
targets as the object file building targets, so the generation of
buildinf.h and similar files would kick in during 'make depend'.

Reviewed-by: Rich Salz <rsalz@openssl.org>