fw: build clean, and check that it stays that way

Two warnings have stood in fw.c since the program was written:

	warning: fw.c:732 auto declared and not used: buf
	warning: fw.c:1286 set and not used: m

Neither matters on its own -- an unused array in fsread, and an m = nil
that the next line overwrites -- but a build that always prints two
warnings is a build whose output nobody reads, which is how the next
one that does matter goes unnoticed.  Both are the sort of thing kencc
tells you for free.

So the suite now builds the source from clean and asks the compiler
whether it had anything to say.  Reintroducing the unused array makes
it fail with the warning printed under the check, which is what a
finding nobody had to look for should look like.  It also checks that
mk succeeded, since a build that does not run produces no warnings
either.  Skipped if the source is not on the machine being tested.

48 pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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