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.
Indeed, will fix.