Determining if the pthread_create exist failed with the following output:
Change Dir: /home/pi/build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_53864/fast"
/usr/bin/make -f CMakeFiles/cmTC_53864.dir/build.make CMakeFiles/cmTC_53864.dir/build
make[1]: Entering directory '/home/pi/build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_53864.dir/CheckSymbolExists.c.o
/usr/bin/cc    -o CMakeFiles/cmTC_53864.dir/CheckSymbolExists.c.o   -c /home/pi/build/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_53864
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_53864.dir/link.txt --verbose=1
/usr/bin/cc      -rdynamic CMakeFiles/cmTC_53864.dir/CheckSymbolExists.c.o  -o cmTC_53864
/usr/bin/ld: CMakeFiles/cmTC_53864.dir/CheckSymbolExists.c.o: in function `main':
CheckSymbolExists.c:(.text+0x38): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
make[1]: *** [CMakeFiles/cmTC_53864.dir/build.make:87: cmTC_53864] Error 1
make[1]: Leaving directory '/home/pi/build/CMakeFiles/CMakeTmp'
make: *** [Makefile:121: cmTC_53864/fast] Error 2

File /home/pi/build/CMakeFiles/CMakeTmp/CheckSymbolExists.c:
/* */
#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/pi/build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_86f8c/fast"
/usr/bin/make -f CMakeFiles/cmTC_86f8c.dir/build.make CMakeFiles/cmTC_86f8c.dir/build
make[1]: Entering directory '/home/pi/build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_86f8c.dir/CheckFunctionExists.c.o
/usr/bin/cc   -DCHECK_FUNCTION_EXISTS=pthread_create   -o CMakeFiles/cmTC_86f8c.dir/CheckFunctionExists.c.o   -c /usr/share/cmake-3.13/Modules/CheckFunctionExists.c
Linking C executable cmTC_86f8c
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_86f8c.dir/link.txt --verbose=1
/usr/bin/cc  -DCHECK_FUNCTION_EXISTS=pthread_create    -rdynamic CMakeFiles/cmTC_86f8c.dir/CheckFunctionExists.c.o  -o cmTC_86f8c -lpthreads
/usr/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
make[1]: *** [CMakeFiles/cmTC_86f8c.dir/build.make:87: cmTC_86f8c] Error 1
make[1]: Leaving directory '/home/pi/build/CMakeFiles/CMakeTmp'
make: *** [Makefile:121: cmTC_86f8c/fast] Error 2