Configure: allow to enable afalgeng if target does not start with Linux

The Debian build system uses a `debian' target which sets CFLAGS and
then we have for instance debian-amd64 which inherits from
linux-x86_64 and debian [0]. So far so good.

Unless there are different suggestions how to do this, I would keep it.
However since the target name does not start with `linux', the build
system does not enable the afalg engine. So in order to get enabled, I
added a
	`enable           => [ "afalgeng" ],'
to the generic linux config which sets it explicit (as suggested by
Richard Levitte). Having this set, we can check for it instead matching
the target name.

[0] https://sources.debian.org/src/openssl/1.1.0g-2/Configurations/20-debian.conf/


Signed-off-by: Sebastian Andrzej Siewior <sebastian@breakpoint.cc>

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5169)