string from a silent implicit non-global substitution to a non-silent
explicit fatal error. Archored substitutions are those containing '^'
or '$'.
The problem with changing the substitution to prevent an infinite
number of matches is that it doesn't provide the necessary feedback
to the user that there's a bug in the/a makefile. Reporting the bug
without making the condition fatal makes the feedback mostly useless
due to the way that make fails to prefix the error with program name,
makefile file name and line number information.
Note that global substitutions of the empty string anchored with '^'
(start of string) or '$' (end of string) do not cause an infinite
number of matches and are therefore not reported and hence are non-
fatal.
Suggested by: bde
Tested with: buildworld