bc: if ^C handling is not selected, there is no interactive mode

bc: if ^C handling is not selected, there is no interactive mode

function                                             old     new   delta
packed_usage                                       33023   33076     +53
bc_num_s                                             252     246      -6
bc_vm_run                                           1859    1850      -9
bc_error_fmt                                          36      20     -16
bc_posix_error_fmt                                    58      41     -17
bc_read_line                                         202     168     -34
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/5 up/down: 53/-82)            Total: -29 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>