Makefile template: Allow separate generation of .pod.in -> .pod

Makefile template: Allow separate generation of .pod.in -> .pod

We do this by adding the attribute 'pod' to all .pod.in -> .pod
generations, like this:

    DEPEND[NAME.pod]{pod}=NAME.pod.in,

...  and selecting out the target files for those dependencies into a
dedicated target 'build_generated_pods', which the 'doc-nits' and
'cmd-nits' make targets are made to depend on.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/14067)