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

Run Build Command:"/usr/bin/make" "cmTC_16e44/fast"
/usr/bin/make -f CMakeFiles/cmTC_16e44.dir/build.make CMakeFiles/cmTC_16e44.dir/build
make[1]: Entering directory '/content/openpose/build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_16e44.dir/CheckSymbolExists.c.o
/usr/bin/cc   -fPIC    -o CMakeFiles/cmTC_16e44.dir/CheckSymbolExists.c.o   -c /content/openpose/build/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_16e44
/usr/local/bin/cmake -E cmake_link_script CMakeFiles/cmTC_16e44.dir/link.txt --verbose=1
/usr/bin/cc -fPIC     -rdynamic CMakeFiles/cmTC_16e44.dir/CheckSymbolExists.c.o  -o cmTC_16e44 
/usr/bin/ld: CMakeFiles/cmTC_16e44.dir/CheckSymbolExists.c.o: in function `main':
CheckSymbolExists.c:(.text+0x1f): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
make[1]: *** [CMakeFiles/cmTC_16e44.dir/build.make:87: cmTC_16e44] Error 1
make[1]: Leaving directory '/content/openpose/build/CMakeFiles/CMakeTmp'
make: *** [Makefile:121: cmTC_16e44/fast] Error 2

File /content/openpose/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: /content/openpose/build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_7ca71/fast"
/usr/bin/make -f CMakeFiles/cmTC_7ca71.dir/build.make CMakeFiles/cmTC_7ca71.dir/build
make[1]: Entering directory '/content/openpose/build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_7ca71.dir/CheckFunctionExists.c.o
/usr/bin/cc   -fPIC -DCHECK_FUNCTION_EXISTS=pthread_create   -o CMakeFiles/cmTC_7ca71.dir/CheckFunctionExists.c.o   -c /usr/local/share/cmake-3.13/Modules/CheckFunctionExists.c
Linking C executable cmTC_7ca71
/usr/local/bin/cmake -E cmake_link_script CMakeFiles/cmTC_7ca71.dir/link.txt --verbose=1
/usr/bin/cc -fPIC -DCHECK_FUNCTION_EXISTS=pthread_create    -rdynamic CMakeFiles/cmTC_7ca71.dir/CheckFunctionExists.c.o  -o cmTC_7ca71 -lpthreads 
/usr/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
make[1]: *** [CMakeFiles/cmTC_7ca71.dir/build.make:87: cmTC_7ca71] Error 1
make[1]: Leaving directory '/content/openpose/build/CMakeFiles/CMakeTmp'
make: *** [Makefile:121: cmTC_7ca71/fast] Error 2