Add makevar SYSCONFDIR, closes #4560

Alright, I've taken a look at this, and I think the best thing to do is the following:

* By default, `make install` tries to put files into `JULIA_HOME/etc/julia/`, and julia only looks for an installation-wide `juliarc.jl` in that folder.  (This is ignoring user-wide `~/.juliarc.jl` files)

* If the user specifies `SYSCONFDIR=blahblahblah`, then `make install` respects that, and updates `build_h.jl` accordingly so that Julia knows where to look at runtime.  To be explicit, `SYSCONFDIR` should equal something like `/etc`, and julia will look in `$SYSCONFDIR/julia/juliarc.jl`, for example.  If nothing is found there, we fallback to the relative path and try that `JULIA_HOME/../etc/julia/juliarc.jl`


@Narrat please review this and see if it solves your problem satisfactorily.  Ideally, package maintainers would simply need to include `SYSCONFDIR=/etc` in their make incantation to get `/etc/julia/juliarc.jl` into the paths that are searched for a startup file.