This will replace the use of Makefile.shared

This also means a small adjustment on how the attributes dso_cflags,
dso_cxxflags and dso_lflags are treated.  They were previously treated
as an extension to shared_cflag, shared_cxxflag and shared_ldflag, but
they should really be regarded as alternatives instead, for example
for darwin, where -dynamiclib is used for shared libraries and -bundle
for DSOs.

We take the opportunity to clean out things that are redundant or
otherwise superfluous (for example the check of GNU ld on platforms
where it never existed).
Reviewed-by: NAndy Polyakov <appro@openssl.org>
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4840)