Performing C SOURCE FILE Test CMAKE_HAVE_LIBC_PTHREAD failed with the following output:
Change Dir: /tmp/instance-1/output-1/build/netopeer2-1.1.53/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_ee05a/fast && make[1]: Entering directory '/tmp/instance-1/output-1/build/netopeer2-1.1.53/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_ee05a.dir/build.make CMakeFiles/cmTC_ee05a.dir/build
make[2]: Entering directory '/tmp/instance-1/output-1/build/netopeer2-1.1.53/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_ee05a.dir/src.c.o
/tmp/instance-1/output-1/host/bin/aarch64-linux-gcc --sysroot=/tmp/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot -D_GNU_SOURCE -D_POSIX_C_SOURCE=200809L -D__BSD_VISIBLE=1  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -DCMAKE_HAVE_LIBC_PTHREAD  -DNDEBUG   -o CMakeFiles/cmTC_ee05a.dir/src.c.o   -c /tmp/instance-1/output-1/build/netopeer2-1.1.53/CMakeFiles/CMakeTmp/src.c
Linking C executable cmTC_ee05a
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_ee05a.dir/link.txt --verbose=1
/tmp/instance-1/output-1/host/bin/aarch64-linux-gcc --sysroot=/tmp/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -DCMAKE_HAVE_LIBC_PTHREAD  -DNDEBUG    CMakeFiles/cmTC_ee05a.dir/src.c.o  -o cmTC_ee05a  /tmp/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot/usr/lib/libssl.so /tmp/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot/usr/lib/libcrypto.so /tmp/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot/usr/lib/libssh.so /tmp/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot/usr/lib/libnetconf2.so 
/tmp/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/aarch64-buildroot-linux-gnu/8.3.0/../../../../aarch64-buildroot-linux-gnu/bin/ld: CMakeFiles/cmTC_ee05a.dir/src.c.o: undefined reference to symbol 'pthread_join@@GLIBC_2.17'
/tmp/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/aarch64-buildroot-linux-gnu/8.3.0/../../../../aarch64-buildroot-linux-gnu/bin/ld: /tmp/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot/lib64/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/cmTC_ee05a.dir/build.make:91: cmTC_ee05a] Error 1
make[2]: Leaving directory '/tmp/instance-1/output-1/build/netopeer2-1.1.53/CMakeFiles/CMakeTmp'
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Source file was:
#include <pthread.h>

void* test_func(void* data)
{
  return data;
}

int main(void)
{
  pthread_t thread;
  pthread_create(&thread, NULL, test_func, NULL);
  pthread_detach(thread);
  pthread_join(thread, NULL);
  pthread_atfork(NULL, NULL, NULL);
  pthread_exit(NULL);

  return 0;
}

Determining if the function pthread_create exists in the pthreads failed with the following output:
Change Dir: /tmp/instance-1/output-1/build/netopeer2-1.1.53/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_aba06/fast && make[1]: Entering directory '/tmp/instance-1/output-1/build/netopeer2-1.1.53/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_aba06.dir/build.make CMakeFiles/cmTC_aba06.dir/build
make[2]: Entering directory '/tmp/instance-1/output-1/build/netopeer2-1.1.53/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_aba06.dir/CheckFunctionExists.c.o
/tmp/instance-1/output-1/host/bin/aarch64-linux-gcc --sysroot=/tmp/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot -D_GNU_SOURCE -D_POSIX_C_SOURCE=200809L -D__BSD_VISIBLE=1  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG   -o CMakeFiles/cmTC_aba06.dir/CheckFunctionExists.c.o   -c /usr/share/cmake-3.16/Modules/CheckFunctionExists.c
Linking C executable cmTC_aba06
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_aba06.dir/link.txt --verbose=1
/tmp/instance-1/output-1/host/bin/aarch64-linux-gcc --sysroot=/tmp/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG    CMakeFiles/cmTC_aba06.dir/CheckFunctionExists.c.o  -o cmTC_aba06  -lpthreads /tmp/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot/usr/lib/libssl.so /tmp/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot/usr/lib/libcrypto.so /tmp/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot/usr/lib/libssh.so /tmp/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot/usr/lib/libnetconf2.so 
/tmp/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/aarch64-buildroot-linux-gnu/8.3.0/../../../../aarch64-buildroot-linux-gnu/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/cmTC_aba06.dir/build.make:91: cmTC_aba06] Error 1
make[2]: Leaving directory '/tmp/instance-1/output-1/build/netopeer2-1.1.53/CMakeFiles/CMakeTmp'
make[1]: *** [Makefile:121: cmTC_aba06/fast] Error 2
make[1]: Leaving directory '/tmp/instance-1/output-1/build/netopeer2-1.1.53/CMakeFiles/CMakeTmp'