Configure - get rid of the special debug_ and release_ settings

Instead, make the build type ("debug" or "release") available through
$config{build_type} and let the configs themselves figure out what the
usual settings (such as "cflags", "lflags" and so on) should be
accordingly.

The benefit with this is that we can now have debug and release
variants of any setting, not just those Configure supports, and may
also involve other factors (the MSVC flags /MD[d] and /MT[d] involve
both build type and whether threading is enabled or not)

Reviewed-by: Andy Polyakov <appro@openssl.org>