CLEANUP: makefile: make the output of the "opts" target more readable

CLEANUP: makefile: make the output of the "opts" target more readable

"make opts" is nice because it shows all options being used, but it
does so in a copy-pastable way that aggregates everything on a single
line, rendering very poorly and making it hard to spot the relevant
variables.

Let's break lines and append a trailing backslash to them instead. This
gives something much more readable which remains copy-pastable. Options
can be inspected and more easily replicated. It was also verified that
copy-pasting the whole block after "make" does continue to work like
before and produces the same output.