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

fixes #8368 @nalimilan
Even better would be to store this as a relative path. The same could be done with SYSCONFDIR. This would allow changing prefix without recompiling anything (which is sometimes useful, and recommended by the GNU guidelines). See have this is done here: https://github.com/JuliaLang/julia/blob/master/Make.inc#L317