Pass MAKEFLAGS down to submake

[wom-bat]

also make system file depend on Makefile variables, so it gets rebuilt if they change.

[wom-bat]

FIxes issue #286

[midnightveil]

I think we need to do this for most examples, yes? But I can do tomorrow if you are busy...

Also won't this not update the values in the file after rerun? I guess it's fine

[dumsum]

Two questions/comments since I've been thinking about this a bit.

1. Is the explicit MAKEFLAGS passing needed? I believe this should already be passed automatically to recursive make calls: https://www.gnu.org/software/make/manual/html_node/Options_002fRecursion.html
   This sDDF commit message also made reference to removing explicit MAKEFLAGS passing for the same reason: au-ts/sddf@e2f87a9

2. If you first run make NFS_SERVER=oldval, and then run make NFS_SERVER=newval, the second invocation should now correctly use newval. But build/Makefile is not regenerated, so a later make -C build will still fall back to the snapshotted oldval. Is that the intended behaviour? (Edit: sorry just realised this was already observed by @midnightveil above)