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: NRich Salz <rsalz@openssl.org>

INSTALL_PREFIX is a confusing name, as there's also --prefix.
Instead, tag along with the rest of the open source world and adopt
the Makefile variable DESTDIR to designate the desired staging
directory.

The Configure option --install_prefix is removed, the only way to
designate a staging directory is with the Makefile variable (this is
also implemented for VMS' descrip.mms et al).
Reviewed-by: NRich Salz <rsalz@openssl.org>

As part of this, change util/mkdef.pl to stop adding libraries to
depend on in its output.  mkdef.pl should ONLY output a symbol
vector.

Because symbol names can't be longer than 31 characters, we use the
compiler to shorten those that are longer down to 23 characters plus
an 8 character CRC.  To make sure users of our header files will pick
up on that automatically, add the DEC C supported extra headers files
__decc_include_prologue.h and __decc_include_epilogue.h.

Furthermore, we add a config.com, so VMS people can configure just as
comfortably as any Unix folks, thusly:

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