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

Run Build Command:"/usr/bin/make" "cmTC_27973/fast"
make[1]: Entering directory '/content/build_linux/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_27973.dir/build.make CMakeFiles/cmTC_27973.dir/build
make[2]: Entering directory '/content/build_linux/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_27973.dir/CheckSymbolExists.c.o
/usr/bin/clang-10    -o CMakeFiles/cmTC_27973.dir/CheckSymbolExists.c.o   -c /content/build_linux/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_27973
/usr/local/lib/python2.7/dist-packages/cmake/data/bin/cmake -E cmake_link_script CMakeFiles/cmTC_27973.dir/link.txt --verbose=1
/usr/bin/clang-10     -static-libstdc++  CMakeFiles/cmTC_27973.dir/CheckSymbolExists.c.o  -o cmTC_27973 
clang: warning: argument unused during compilation: '-static-libstdc++' [-Wunused-command-line-argument]
CMakeFiles/cmTC_27973.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text+0x19): undefined reference to `pthread_create'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
CMakeFiles/cmTC_27973.dir/build.make:86: recipe for target 'cmTC_27973' failed
make[2]: *** [cmTC_27973] Error 1
make[2]: Leaving directory '/content/build_linux/CMakeFiles/CMakeTmp'
Makefile:121: recipe for target 'cmTC_27973/fast' failed
make[1]: *** [cmTC_27973/fast] Error 2
make[1]: Leaving directory '/content/build_linux/CMakeFiles/CMakeTmp'

File /content/build_linux/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/build_linux/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_fa7db/fast"
make[1]: Entering directory '/content/build_linux/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_fa7db.dir/build.make CMakeFiles/cmTC_fa7db.dir/build
make[2]: Entering directory '/content/build_linux/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_fa7db.dir/CheckFunctionExists.c.o
/usr/bin/clang-10   -DCHECK_FUNCTION_EXISTS=pthread_create   -o CMakeFiles/cmTC_fa7db.dir/CheckFunctionExists.c.o   -c /usr/local/lib/python2.7/dist-packages/cmake/data/share/cmake-3.12/Modules/CheckFunctionExists.c
Linking C executable cmTC_fa7db
/usr/local/lib/python2.7/dist-packages/cmake/data/bin/cmake -E cmake_link_script CMakeFiles/cmTC_fa7db.dir/link.txt --verbose=1
/usr/bin/clang-10  -DCHECK_FUNCTION_EXISTS=pthread_create   -static-libstdc++  CMakeFiles/cmTC_fa7db.dir/CheckFunctionExists.c.o  -o cmTC_fa7db -lpthreads 
clang: warning: argument unused during compilation: '-static-libstdc++' [-Wunused-command-line-argument]
/usr/bin/ld: cannot find -lpthreads
clang: error: linker command failed with exit code 1 (use -v to see invocation)
CMakeFiles/cmTC_fa7db.dir/build.make:86: recipe for target 'cmTC_fa7db' failed
make[2]: *** [cmTC_fa7db] Error 1
make[2]: Leaving directory '/content/build_linux/CMakeFiles/CMakeTmp'
Makefile:121: recipe for target 'cmTC_fa7db/fast' failed
make[1]: *** [cmTC_fa7db/fast] Error 2
make[1]: Leaving directory '/content/build_linux/CMakeFiles/CMakeTmp'