Build issues with GCC-15: many incompatible-pointer-types errors

[NHOrus]

GCC 15 will default to -std=gnu23. C23 removes unprotoyped functions, so x() now means x(void)

On Gentoo:

x86_64-pc-linux-gnu-gcc -O2 -pipe -march=native -ggdb -I. -Wall -I/usr/include/glib-2.0 -I/usr/lib64/glib-2.0/include  -Wno-stringop-truncation -Wmissing-prototypes -Wmissing-declarations -Wformat-security    -c -o atopsar.o atopsar.c
showprocs.c: In function ‘showprocline’:
showprocs.c:500:38: error: too many arguments to function ‘curelem.pf->doactiveconvert’; expected 0, have 3
  500 |                         printg("%s", curelem.pf->doactiveconvert(curstat,
      |                                      ^~~~~~~                     ~~~~~~~
In file included from showprocs.c:61:
showlinux.h:105:17: note: declared here
  105 |         char *(*doactiveconvert)();      // pointer to conv function
      |                 ^~~~~~~~~~~~~~~
showprocs.c:505:38: error: too many arguments to function ‘curelem.pf->doexitconvert’; expected 0, have 3
  505 |                         printg("%s", curelem.pf->doexitconvert(curstat,
      |                                      ^~~~~~~                   ~~~~~~~
showlinux.h:107:17: note: declared here
  107 |         char *(*doexitconvert)();        // pointer to conv function
      |                 ^~~~~~~~~~~~~
showprocs.c: At top level:
showprocs.c:582:20: error: initialization of ‘char * (*)(void)’ from incompatible pointer type ‘char * (*)(struct tstat *, int,  int)’ [-Wincompatible-pointer-types]
  582 |    { "TID", "TID", procprt_TID_ae, procprt_TID_ae, ' ', 5}; //DYNAMIC WIDTH!
      |                    ^~~~~~~~~~~~~~
showprocs.c:582:20: note: (near initialization for ‘procprt_TID.doactiveconvert’)
showprocs.c:570:1: note: ‘procprt_TID_ae’ declared here
  570 | procprt_TID_ae(struct tstat *curstat, int avgval, int nsecs)
      | ^~~~~~~~~~~~~~
showprocs.c:582:36: error: initialization of ‘char * (*)(void)’ from incompatible pointer type ‘char * (*)(struct tstat *, int,  int)’ [-Wincompatible-pointer-types]
  582 |    { "TID", "TID", procprt_TID_ae, procprt_TID_ae, ' ', 5}; //DYNAMIC WIDTH!
      |                                    ^~~~~~~~~~~~~~
showprocs.c:582:36: note: (near initialization for ‘procprt_TID.doexitconvert’)
showprocs.c:570:1: note: ‘procprt_TID_ae’ declared here
  570 | procprt_TID_ae(struct tstat *curstat, int avgval, int nsecs)
      | ^~~~~~~~~~~~~~
showprocs.c:606:20: error: initialization of ‘char * (*)(void)’ from incompatible pointer type ‘char * (*)(struct tstat *, int,  int)’ [-Wincompatible-pointer-types]
  606 |    { "PID", "PID", procprt_PID_a, procprt_PID_e, ' ', 5}; //DYNAMIC WIDTH!
      |                    ^~~~~~~~~~~~~
showprocs.c:606:20: note: (near initialization for ‘procprt_PID.doactiveconvert’)
showprocs.c:585:1: note: ‘procprt_PID_a’ declared here
  585 | procprt_PID_a(struct tstat *curstat, int avgval, int nsecs)

see https://bugs.gentoo.org/945250