[PD] error: ‘t_signal’ {aka ‘struct _signal’} has no member named ‘s_length’

Working on an object of mine, I copied code from snake_in~ and snake_out~ to enable multi-channel I/Os, but I get errors on s_length and s_nchans. More specifically, I get the following:

src/neuralnet~.c:4525:94: error: ‘t_signal’ {aka ‘struct _signal’} has 
no member named ‘s_length’
  4525 |                         dsp_add_copy(sp[i]->s_vec, 
sp[x->x_nmultichans_in]->s_vec + i * sp[0]->s_length, sp[0]->s_length);
| ^~
src/neuralnet~.c:4525:111: error: ‘t_signal’ {aka ‘struct _signal’} has 
no member named ‘s_length’
  4525 |                         dsp_add_copy(sp[i]->s_vec, 
sp[x->x_nmultichans_in]->s_vec + i * sp[0]->s_length, sp[0]->s_length);
| ^~
src/neuralnet~.c:4534:62: error: ‘t_signal’ {aka ‘struct _signal’} has 
no member named ‘s_nchans’
  4534 |                 int usenchans = (x->x_nmultichans_out < 
sp[0]->s_nchans ? x->x_nmultichans_out : sp[0]->s_nchans);
|                                                              ^~
src/neuralnet~.c:4534:103: error: ‘t_signal’ {aka ‘struct _signal’} has 
no member named ‘s_nchans’
  4534 |                 int usenchans = (x->x_nmultichans_out < 
sp[0]->s_nchans ? x->x_nmultichans_out : sp[0]->s_nchans);
| ^~
src/neuralnet~.c:4539:70: error: ‘t_signal’ {aka ‘struct _signal’} has 
no member named ‘s_length’
  4539 | dsp_add_copy(sp[0]->s_vec + i * sp[0]->s_length, 
sp[i+1]->s_vec, sp[0]->s_length);
| ^~
src/neuralnet~.c:4539:103: error: ‘t_signal’ {aka ‘struct _signal’} has 
no member named ‘s_length’
  4539 | dsp_add_copy(sp[0]->s_vec + i * sp[0]->s_length, 
sp[i+1]->s_vec, sp[0]->s_length);
| ^~
src/neuralnet~.c:4542:67: error: ‘t_signal’ {aka ‘struct _signal’} has 
no member named ‘s_length’
  4542 | dsp_add_zero(sp[i+1]->s_vec, sp[0]->s_length);
| ^~
src/neuralnet~.c: In function ‘neuralnet_tilde_setup’:
src/neuralnet~.c:4944:52: error: ‘CLASS_MULTICHANNEL’ undeclared (first 
use in this function)
  4944 |                         sizeof(t_neuralnet_tilde), 
CLASS_MULTICHANNEL, A_GIMME, 0);

Is it likely that the Makefile is looking at an m_pd.h older than 0-54.0? I have installed 0-55.1 by compiling it. Is there something I need to do to point the compiler to the correct m_pd.h file?