Build error: `fft_cmplx.c:192:12: error: call to undeclared function 'fft_krn'; ISO C99 and later do not support implicit function declarations`

[barracuda156]

  /opt/local/var/macports/build/_Users_runner_work_macports-ports_macports-ports_ports_science_dspl/dspl/work/compwrap/cc/usr/bin/clang -c -fPIC -Wall -O3 -I../include -Isrc -DBUILD_LIB -DLINUX_OS  src/dft/fft_free.c -o obj/fft_free.o -lm 
  clang: warning: -lm: 'linker' input unused [-Wunused-command-line-argument]
  clang: warning: -lm: 'linker' input unused [-Wunused-command-line-argument]
  src/dft/fft_cmplx.c:192:12: error: call to undeclared function 'fft_krn'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
      return fft_krn(pfft->t1, y, pfft, n, 0);
             ^
  1 error generated.
  make[1]: *** [obj/fft_cmplx.o] Error 1
  make[1]: *** Waiting for unfinished jobs....

Is there some header missing?

gcc-13 still builds it, spitting out a warning; clang just fails.

[ryandesign]

Specifically, Apple clang 12 and later and llvm.org clang 16 and later consider this to be an error.