Test struct existance against sizeof() operator

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).