tack: --enable-warnings configure flag doesn't actually work

[Samuel Bronson]

I feel a little silly not to have noticed this sooner, but it occurred
to me today as I was working on packaging the latest snapshot that the
warning flags that tack's configure script detects when given the
--enable-warnings flag don't actually get passed to the compiler.

Searching through config.status reveals that it is prepared to
substitute them in wherever @EXTRA_CFLAGS@ appears in Makefile.in, but
Makefile.in doesn't actually contain that anywhere.

And if you do add it in, you get a whole pile of warnings about
undefined preprocessor macros...

(Also, configure.in looks really strange now; it's best not to use TAB
for any visual effect besides "some indentation at the beginning of
the line" or "skip to the next 8th column".)