[GCC15/C23] pdf.c:79:32: error: passing argument 2 of ‘start_process’ from incompatible pointer type

[immolo]

Describe the bug
Upcoming GCC 15 defaults to -std=gnu23. cups-filters=2.01 fails to build as
follows:

filter/foomatic-rip/pdf.c:79:32: error: passing argument 2 of ‘start_process’ from incompatible pointer type [-Wincompatible-pointer-types]
   79 |   kid3 = start_process("kid3", exec_kid3, (void *)cmd, NULL, NULL);
      |                                ^~~~~~~~~
      |                                |
      |                                int (*)(FILE *, FILE *, void *)
In file included from filter/foomatic-rip/pdf.c:16:
filter/foomatic-rip/process.h:21:45: note: expected ‘int (*)(void)’ but argument is of type ‘int (*)(FILE *, FILE *, void *)’
   21 | pid_t start_process(const char *name, int (*proc_func)(), void *user_arg,
      |                                       ~~~~~~^~~~~~~~~~~~
filter/foomatic-rip/renderer.c: In function ‘exec_kid3’:
filter/foomatic-rip/renderer.c:456:32: error: passing argument 2 of ‘start_process’ from incompatible pointer type [-Wincompatible-pointer-types]
  456 |   kid4 = start_process("kid4", exec_kid4, NULL, &kid4in, NULL);
      |                                ^~~~~~~~~
      |                                |
      |                                int (*)(FILE *, FILE *, void *)
In file included from filter/foomatic-rip/renderer.c:21:
filter/foomatic-rip/process.h:21:45: note: expected ‘int (*)(void)’ but argument is of type ‘int (*)(FILE *, FILE *, void *)’
   21 | pid_t start_process(const char *name, int (*proc_func)(), void *user_arg,
      |                                       ~~~~~~^~~~~~~~~~~~
make[2]: *** [Makefile:2047: filter/foomatic-rip/foomatic_rip-pdf.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[2]: *** [Makefile:2089: filter/foomatic-rip/foomatic_rip-renderer.o] Error 1
filter/foomatic-rip/postscript.c: In function ‘get_renderer_handle’:
filter/foomatic-rip/postscript.c:1444:32: error: passing argument 2 of ‘start_process’ from incompatible pointer type [-Wincompatible-pointer-types]
 1444 |   kid3 = start_process("kid3", exec_kid3, (void *)cmdline->data, &kid3in, NULL);
      |                                ^~~~~~~~~
      |                                |
      |                                int (*)(FILE *, FILE *, void *)
In file included from filter/foomatic-rip/postscript.c:17:
filter/foomatic-rip/process.h:21:45: note: expected ‘int (*)(void)’ but argument is of type ‘int (*)(FILE *, FILE *, void *)’
   21 | pid_t start_process(const char *name, int (*proc_func)(), void *user_arg,
      |                                       ~~~~~~^~~~~~~~~~~~
make[2]: *** [Makefile:2061: filter/foomatic-rip/foomatic_rip-postscript.o] Error 1
filter/foomatic-rip/process.c:231:1: error: conflicting types for ‘start_process’; have ‘pid_t(const char *, int (*)(FILE *, FILE *, void *), void *, FILE **, FILE **)’ {aka ‘int(const char *, int (*)(FILE *, FILE *, void *), void *, FILE **, FILE **)’}
  231 | start_process(const char *name,
      | ^~~~~~~~~~~~~
In file included from filter/foomatic-rip/process.c:14:
filter/foomatic-rip/process.h:21:7: note: previous declaration of ‘start_process’ with type ‘pid_t(const char *, int (*)(void), void *, FILE **, FILE **)’ {aka ‘int(const char *, int (*)(void), void *, FILE **, FILE **)’}
   21 | pid_t start_process(const char *name, int (*proc_func)(), void *user_arg,

This is because C23 removes unprotoyped functions.

It should be possible to reproduce these with older GCC (and Clang) with
-std=gnu23 or -std=c23 set manually.

tdr-emerge-info.txt
cups-filters-2.0.1.buid.log

Gentoo bug: https://bugs.gentoo.org/944074