Instead of declaring an "unused-variable" or "unused-but-set-variable"
structure.

This avoid unexpected broken configurations when using something like:
$ ./configure CFLAGS="-Wall -Wextra -Werror"
It causes AC_COMPILE_IFELSE to fail and thus leading to a mismatch
configuration (because the CFLAGS are passed to the macro).

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(4d7b4c5526c5)

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Client side is not complete.

and /**/ comments