Build failure against Clang 13

[Alexey Dokuchaev via Bugs and suggestions for GNU APL]

Hi there,

GNU APL configures itself to build with -Werror by default (which is good
thing), and now fails to build with Clang 13.0.0:

  Bif_F12_FORMAT.cc:579:5: error: variable 'sum' set but not used
  [-Werror,-Wunused-but-set-variable]
  int sum = 0;
      ^
  1 error generated.

./danfe