Make install cleaning and improvements

The first commit makes the tree resulting from `make install` cleaner.

The second one is an attempt at fixing https://github.com/JuliaLang/julia/issues/8367, but it currently doesn't work. I'd appreciate some help from the genius who wrote Makefile:27. It should be adapted so that the symlink to `doc/` is no longer created at `$(build_datarootdir)/julia/doc`, but at `$(build_docdir)` -- Make.inc:102 should then also be made consistent with Make.inc:89.
I'm not sure what this line is doing.  Any ideas?  I see that it's a transcription of [this line](https://github.com/nalimilan/julia/blob/3113bfd0aa54c47917b2b99d6d477361d47a8597/Make.inc#L761) but I don't know what it means.