Fix the web of Makefile cross-references across subdirs

[jimklimov]

• We had issues (links welcome) with builds of parts of codebase from sub-directories, rather than builds arranged normally from the root directory. Large part of the mess was the absence of dependencies on files (e.g. *.la archives) located in other sub-directories, so those were not automatically built when we wanted to e.g. configure && cd tests && make. Now such situations should resolve correctly.

[jimklimov]

Note for posterity: at least one build on a Debian 8 derived environment ended up creating directories literally named $(top_srcdir) and could not resolve some *.Plo files for the 4 sources impacted here, while compiling nut-scanner.

Commenting those 4 resulting include lines away from generated tools/nut-scanner/Makefile helped complete that build.

This was not seen anywhere else so I'd say it was either a strange system or some bug in older autotools.

But if this bites someone else, posted issues or better yet PRs are welcome. Returning to explicit ../ paths might be an option, but "really fixing" the issue for those and modern toolkits would be better.