c99 lets us do neat stuff like:

    {
      int j, k;
      foo(&j, &k);
      int z = j + k;
    }

and also
    struct point { int x; int y; };
    struct point pt = { .x=5, .y=5 };

This commit makes the configure scripts check to make sure your
compiler implements them.  It also disables our longstanding warning
about midblock declarations.

Closes ticket 13233.