Determining if the pthread_create exist failed with the following output:
Change Dir: /usr/AMBER-Files/amber20_src/build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_fd407/fast"
/usr/bin/make -f CMakeFiles/cmTC_fd407.dir/build.make CMakeFiles/cmTC_fd407.dir/build
make[1]: Entering directory '/usr/AMBER-Files/amber20_src/build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_fd407.dir/CheckSymbolExists.c.o
/usr/bin/gcc   -fPIC    -std=gnu99 -o CMakeFiles/cmTC_fd407.dir/CheckSymbolExists.c.o   -c /usr/AMBER-Files/amber20_src/build/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_fd407
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_fd407.dir/link.txt --verbose=1
/usr/bin/gcc -fPIC     CMakeFiles/cmTC_fd407.dir/CheckSymbolExists.c.o  -o cmTC_fd407 
CMakeFiles/cmTC_fd407.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text+0x1b): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_fd407.dir/build.make:97: recipe for target 'cmTC_fd407' failed
make[1]: *** [cmTC_fd407] Error 1
make[1]: Leaving directory '/usr/AMBER-Files/amber20_src/build/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_fd407/fast' failed
make: *** [cmTC_fd407/fast] Error 2

File /usr/AMBER-Files/amber20_src/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: /usr/AMBER-Files/amber20_src/build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_9df28/fast"
/usr/bin/make -f CMakeFiles/cmTC_9df28.dir/build.make CMakeFiles/cmTC_9df28.dir/build
make[1]: Entering directory '/usr/AMBER-Files/amber20_src/build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_9df28.dir/CheckFunctionExists.c.o
/usr/bin/gcc   -fPIC -DCHECK_FUNCTION_EXISTS=pthread_create   -std=gnu99 -o CMakeFiles/cmTC_9df28.dir/CheckFunctionExists.c.o   -c /usr/share/cmake-3.10/Modules/CheckFunctionExists.c
Linking C executable cmTC_9df28
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_9df28.dir/link.txt --verbose=1
/usr/bin/gcc -fPIC -DCHECK_FUNCTION_EXISTS=pthread_create    CMakeFiles/cmTC_9df28.dir/CheckFunctionExists.c.o  -o cmTC_9df28 -lpthreads 
/usr/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_9df28.dir/build.make:97: recipe for target 'cmTC_9df28' failed
make[1]: *** [cmTC_9df28] Error 1
make[1]: Leaving directory '/usr/AMBER-Files/amber20_src/build/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_9df28/fast' failed
make: *** [cmTC_9df28/fast] Error 2