platform->sharedlib_simple(): return undef when same as platform->sharedlib()

On some Unix-like platforms, there is no such thing as versioned shared
libraries.  In this case, platform->sharedlib_simple() should simply
return undef.  Among others, this avoids the shared libraries to be
installed as symlinks on themselves.

Fixes #16012

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/16023)

Configurations/platform/Unix.pm

@@ -63,6 +63,7 @@ sub sharedname_simple {
 }
 
 sub sharedlib_simple {
+    return undef if $_[0]->shlibext() eq $_[0]->shlibextsimple();
     return platform::BASE::__concat($_[0]->sharedname_simple($_[1]),
                                     $_[0]->shlibextsimple());
 }