Determining if the strnstr exist failed with the following output:
Change Dir: /var/tmp/portage/net-misc/netopeer2-2.1.23/work/netopeer2-2.1.23_build/CMakeFiles/CMakeScratch/TryCompile-u7ufWY

Run Build Command(s):/usr/bin/ninja cmTC_50241 && [1/2] Building C object CMakeFiles/cmTC_50241.dir/CheckSymbolExists.c.o
FAILED: CMakeFiles/cmTC_50241.dir/CheckSymbolExists.c.o 
/usr/bin/x86_64-pc-linux-gnu-gcc -D_GNU_SOURCE -D_POSIX_C_SOURCE=200809L -D__BSD_VISIBLE=1   -O2 -march=x86-64 -pipe -pipe -frecord-gcc-switches -fno-diagnostics-color -fmessage-length=0 -Wall -Wextra -std=gnu99 -o CMakeFiles/cmTC_50241.dir/CheckSymbolExists.c.o -c /var/tmp/portage/net-misc/netopeer2-2.1.23/work/netopeer2-2.1.23_build/CMakeFiles/CMakeScratch/TryCompile-u7ufWY/CheckSymbolExists.c
/var/tmp/portage/net-misc/netopeer2-2.1.23/work/netopeer2-2.1.23_build/CMakeFiles/CMakeScratch/TryCompile-u7ufWY/CheckSymbolExists.c: In function ‘main’:
/var/tmp/portage/net-misc/netopeer2-2.1.23/work/netopeer2-2.1.23_build/CMakeFiles/CMakeScratch/TryCompile-u7ufWY/CheckSymbolExists.c:8:19: error: ‘strnstr’ undeclared (first use in this function); did you mean ‘strstr’?
    8 |   return ((int*)(&strnstr))[argc];
      |                   ^~~~~~~
      |                   strstr
/var/tmp/portage/net-misc/netopeer2-2.1.23/work/netopeer2-2.1.23_build/CMakeFiles/CMakeScratch/TryCompile-u7ufWY/CheckSymbolExists.c:8:19: note: each undeclared identifier is reported only once for each function it appears in
/var/tmp/portage/net-misc/netopeer2-2.1.23/work/netopeer2-2.1.23_build/CMakeFiles/CMakeScratch/TryCompile-u7ufWY/CheckSymbolExists.c:4:14: warning: parameter ‘argc’ set but not used [-Wunused-but-set-parameter]
    4 | int main(int argc, char** argv)
      |          ~~~~^~~~
ninja: build stopped: subcommand failed.


File CheckSymbolExists.c:
/* */
#include <string.h>

int main(int argc, char** argv)
{
  (void)argv;
#ifndef strnstr
  return ((int*)(&strnstr))[argc];
#else
  (void)argc;
  return 0;
#endif
}