This allows tracking down what piece of code is outputting stuff in
the echo area.
* eval.c (call_debugger): Make the function non-static so that we
can call it from set_message.
* xdisp.c (set_message): Implement the new variable `debug-on-message'.
(syms_of_xdisp): Defvar it and `inhibit-debug-on-message'.
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