I just come across the --trace flag to gnumake (added in 4.0) which I suspect might have been very useful in all sorts of circumstances debugging the build rules if I’d known about it:

From the NEWS file:

* New command line option: --trace enables tracing of targets.  When enabled
  the recipe to be invoked is printed even if it would otherwise be suppressed
  by .SILENT or a "@" prefix character.  Also before each recipe is run the
  makefile name and linenumber where it was defined are shown as well as the
  prerequisites that caused the target to be considered out of date.

@Michael, when you next get a build loop, please try adding this flag to help us figure out what’s going on.

- Andrew