add DATAROOTDIR to build_h.jl, use it in Base.runtests()

fixes #8368 @nalimilan
I was going based on the example where `SYSCONFDIR` gets used here https://github.com/JuliaLang/julia/blob/15424e37d941e166206803ed6de8f31a44c48027/base/client.jl#L322

Unfortunately prefix is usually empty while `base/Makefile` is getting executed, since the default setting for prefix is here https://github.com/JuliaLang/julia/blob/15424e37d941e166206803ed6de8f31a44c48027/Makefile#L214 in the top-level Makefile, rather than in Make.inc.

So in a normal default source build, `Base.SYSCONFDIR` is currently `/etc` which isn't so useful.