Makefile.in: turn of -Wsign-compare for bison output

conffile.yy.c: In function `yy_get_next_buffer':
conffile.yy.c:1228:44: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
  if ((int) ((yy_n_chars) + number_to_move) > YY_CURRENT_BUFFER_LVALUE->yy_buf_size) {
                                            ^

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>