Build: make it possibly to specify subdirs in build.info

This adds a keyword SUBDIRS for build.info, to be used like this:

    SUBDIRS=foo bar

This tells Configure that it should look for 'build.info' in the
relative subdirectories 'foo' and 'bar' as well.

Reviewed-by: Tim Hudson <tjh@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/7558)