Determining if the strnstr exist failed with the following output:
Change Dir: /home/buildroot/autobuild/instance-0/output-1/build/libyang-2.0.112/buildroot-build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_977e3/fast && make[1]: Entering directory '/home/buildroot/autobuild/instance-0/output-1/build/libyang-2.0.112/buildroot-build/CMakeFiles/CMakeTmp'
/usr/bin/make  -f CMakeFiles/cmTC_977e3.dir/build.make CMakeFiles/cmTC_977e3.dir/build
make[2]: Entering directory '/home/buildroot/autobuild/instance-0/output-1/build/libyang-2.0.112/buildroot-build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_977e3.dir/CheckSymbolExists.c.o
/home/buildroot/autobuild/instance-0/output-1/host/bin/riscv64-linux-gcc --sysroot=/home/buildroot/autobuild/instance-0/output-1/host/riscv64-buildroot-linux-musl/sysroot -D_GNU_SOURCE -D_POSIX_C_SOURCE=200809L -D__BSD_VISIBLE=1  -g3 -O0 -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -O2 -g0 -Wall -Wextra -Wpedantic -std=c11    -o CMakeFiles/cmTC_977e3.dir/CheckSymbolExists.c.o   -c /home/buildroot/autobuild/instance-0/output-1/build/libyang-2.0.112/buildroot-build/CMakeFiles/CMakeTmp/CheckSymbolExists.c
/home/buildroot/autobuild/instance-0/output-1/build/libyang-2.0.112/buildroot-build/CMakeFiles/CMakeTmp/CheckSymbolExists.c: In function 'main':
/home/buildroot/autobuild/instance-0/output-1/build/libyang-2.0.112/buildroot-build/CMakeFiles/CMakeTmp/CheckSymbolExists.c:8:19: error: 'strnstr' undeclared (first use in this function); did you mean 'strstr'?
    8 |   return ((int*)(&strnstr))[argc];
      |                   ^~~~~~~
      |                   strstr
/home/buildroot/autobuild/instance-0/output-1/build/libyang-2.0.112/buildroot-build/CMakeFiles/CMakeTmp/CheckSymbolExists.c:8:19: note: each undeclared identifier is reported only once for each function it appears in
/home/buildroot/autobuild/instance-0/output-1/build/libyang-2.0.112/buildroot-build/CMakeFiles/CMakeTmp/CheckSymbolExists.c:4:14: warning: parameter 'argc' set but not used [-Wunused-but-set-parameter]
    4 | int main(int argc, char** argv)
      |          ~~~~^~~~
CMakeFiles/cmTC_977e3.dir/build.make:68: recipe for target 'CMakeFiles/cmTC_977e3.dir/CheckSymbolExists.c.o' failed
make[2]: *** [CMakeFiles/cmTC_977e3.dir/CheckSymbolExists.c.o] Error 1
make[2]: Leaving directory '/home/buildroot/autobuild/instance-0/output-1/build/libyang-2.0.112/buildroot-build/CMakeFiles/CMakeTmp'
Makefile:124: recipe for target 'cmTC_977e3/fast' failed
make[1]: *** [cmTC_977e3/fast] Error 2
make[1]: Leaving directory '/home/buildroot/autobuild/instance-0/output-1/build/libyang-2.0.112/buildroot-build/CMakeFiles/CMakeTmp'


File /home/buildroot/autobuild/instance-0/output-1/build/libyang-2.0.112/buildroot-build/CMakeFiles/CMakeTmp/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
}