Automatic 'make depend' for the unified build scheme

This isn't the fully featured combination of compiler generated
dependency files and Makefile include directives, but a cheaper
variant of the same.

The dependency files are generated automatically, but then we have the
usual "depend" target.  However, we depend on it in the bigger phony
targets that are the most likely to be used.  That make this feature
automatic enough.

A side effect is that we can't use the build file's timestamp to check
if reconfiguring might be in order.  In its place, we use a flag file
that depends on Configure and the build file template and depend on it
in spots where it makes sense to check for the need to reconfigure.

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