Remove all special make depend flags, as well as OPENSSL_DOING_MAKEDEPEND

All those flags existed because we had all the dependencies versioned
in the repository, and wanted to have it be consistent, no matter what
the local configuration was.  Now that the dependencies are gone from
the versioned Makefile.ins, it makes much more sense to use the exact
same flags as when compiling the object files.

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