new stdbool test provokes warning

[Jim Meyering]

Hi Bruno,

The new stdbool test causes this warning/failure in coreutils:

    cc1: warnings being treated as errors
    test-stdbool.c: In function 'main':
    test-stdbool.c:98:9: error: unused variable 'e1' [-Wunused-variable]
    make[8]: *** [test-stdbool.o] Error 1

Here's the code:

     main ()
     {
     #if HAVE_STDBOOL_H || defined __GNUC__ /* See above.  */
     # ifdef ADDRESS_CHECK_OKAY /* Avoid gcc warning.  */
       /* A cast from a variable's address to bool is valid in expressions.  */
       bool e1 = &s;
     # endif
     #endif


One way to avoid those is to add an

  (void) e1;

statement after each (there's another, above),
but that might not be enough for some compilers.
To go all the way, we could use ignore_value.

Do you have a preference -- or another approach?