Determining if the pthread_create exist failed with the following output:
Change Dir: /home/naourr/work/instance-1/output-1/build/spdlog-1.4.2/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/gmake cmTC_45317/fast 
gmake[1]: Entering directory '/home/naourr/work/instance-1/output-1/build/spdlog-1.4.2/CMakeFiles/CMakeTmp'
/usr/bin/gmake -f CMakeFiles/cmTC_45317.dir/build.make CMakeFiles/cmTC_45317.dir/build
gmake[2]: Entering directory '/home/naourr/work/instance-1/output-1/build/spdlog-1.4.2/CMakeFiles/CMakeTmp'
Building CXX object CMakeFiles/cmTC_45317.dir/CheckSymbolExists.cxx.o
/home/naourr/work/instance-1/output-1/host/bin/aarch64-none-linux-gnu-g++ --sysroot=/home/naourr/work/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot    -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG   -o CMakeFiles/cmTC_45317.dir/CheckSymbolExists.cxx.o -c /home/naourr/work/instance-1/output-1/build/spdlog-1.4.2/CMakeFiles/CMakeTmp/CheckSymbolExists.cxx
Linking CXX executable cmTC_45317
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_45317.dir/link.txt --verbose=1
/home/naourr/work/instance-1/output-1/host/bin/aarch64-none-linux-gnu-g++ --sysroot=/home/naourr/work/instance-1/output-1/host/aarch64-buildroot-linux-gnu/sysroot  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG    CMakeFiles/cmTC_45317.dir/CheckSymbolExists.cxx.o  -o cmTC_45317 
/home/naourr/work/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/aarch64-none-linux-gnu/9.2.1/../../../../aarch64-none-linux-gnu/bin/ld: CMakeFiles/cmTC_45317.dir/CheckSymbolExists.cxx.o: in function `main':
CheckSymbolExists.cxx:(.text.startup+0x0): undefined reference to `pthread_create'
/home/naourr/work/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/aarch64-none-linux-gnu/9.2.1/../../../../aarch64-none-linux-gnu/bin/ld: CheckSymbolExists.cxx:(.text.startup+0x4): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
gmake[2]: *** [CMakeFiles/cmTC_45317.dir/build.make:87: cmTC_45317] Error 1
gmake[2]: Leaving directory '/home/naourr/work/instance-1/output-1/build/spdlog-1.4.2/CMakeFiles/CMakeTmp'
gmake[1]: *** [Makefile:121: cmTC_45317/fast] Error 2
gmake[1]: Leaving directory '/home/naourr/work/instance-1/output-1/build/spdlog-1.4.2/CMakeFiles/CMakeTmp'

File /home/naourr/work/instance-1/output-1/build/spdlog-1.4.2/CMakeFiles/CMakeTmp/CheckSymbolExists.cxx:
/* */
#include <pthread.h>

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

Determining if the function pthread_create exists in the pthreads failed with the following output:
Change Dir: /home/naourr/work/instance-1/output-1/build/spdlog-1.4.2/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/gmake cmTC_1561d/fast 
gmake[1]: Entering directory '/home/naourr/work/instance-1/output-1/build/spdlog-1.4.2/CMakeFiles/CMakeTmp'
/usr/bin/gmake -f CMakeFiles/cmTC_1561d.dir/build.make CMakeFiles/cmTC_1561d.dir/build
gmake[2]: Entering directory '/home/naourr/work/instance-1/output-1/build/spdlog-1.4.2/CMakeFiles/CMakeTmp'
Building CXX object CMakeFiles/cmTC_1561d.dir/CheckFunctionExists.cxx.o
/home/naourr/work/instance-1/output-1/host/bin/aarch64-none-linux-gnu-g++ --sysroot=/home/naourr/work/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   -o CMakeFiles/cmTC_1561d.dir/CheckFunctionExists.cxx.o -c /home/naourr/work/instance-1/output-1/build/spdlog-1.4.2/CMakeFiles/CheckLibraryExists/CheckFunctionExists.cxx
Linking CXX executable cmTC_1561d
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_1561d.dir/link.txt --verbose=1
/home/naourr/work/instance-1/output-1/host/bin/aarch64-none-linux-gnu-g++ --sysroot=/home/naourr/work/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_1561d.dir/CheckFunctionExists.cxx.o  -o cmTC_1561d -lpthreads 
/home/naourr/work/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/aarch64-none-linux-gnu/9.2.1/../../../../aarch64-none-linux-gnu/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
gmake[2]: *** [CMakeFiles/cmTC_1561d.dir/build.make:87: cmTC_1561d] Error 1
gmake[2]: Leaving directory '/home/naourr/work/instance-1/output-1/build/spdlog-1.4.2/CMakeFiles/CMakeTmp'
gmake[1]: *** [Makefile:121: cmTC_1561d/fast] Error 2
gmake[1]: Leaving directory '/home/naourr/work/instance-1/output-1/build/spdlog-1.4.2/CMakeFiles/CMakeTmp'