Performing C SOURCE FILE Test CMAKE_HAVE_LIBC_PTHREAD failed with the following output:
Change Dir: /home/pi/mjpg-streamer/mjpg-streamer-experimental/_build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/gmake cmTC_a0dd7/fast && gmake[1]: Entering directory '/home/pi/mjpg-streamer/mjpg-streamer-experimental/_build/CMakeFiles/CMakeTmp'
/usr/bin/gmake  -f CMakeFiles/cmTC_a0dd7.dir/build.make CMakeFiles/cmTC_a0dd7.dir/build
gmake[2]: Entering directory '/home/pi/mjpg-streamer/mjpg-streamer-experimental/_build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_a0dd7.dir/src.c.o
/usr/bin/cc   -DCMAKE_HAVE_LIBC_PTHREAD -o CMakeFiles/cmTC_a0dd7.dir/src.c.o -c /home/pi/mjpg-streamer/mjpg-streamer-experimental/_build/CMakeFiles/CMakeTmp/src.c
Linking C executable cmTC_a0dd7
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_a0dd7.dir/link.txt --verbose=1
/usr/bin/cc  -DCMAKE_HAVE_LIBC_PTHREAD -rdynamic CMakeFiles/cmTC_a0dd7.dir/src.c.o -o cmTC_a0dd7 
/usr/bin/ld: CMakeFiles/cmTC_a0dd7.dir/src.c.o: in function `main':
src.c:(.text+0x40): undefined reference to `pthread_create'
/usr/bin/ld: src.c:(.text+0x4c): undefined reference to `pthread_detach'
/usr/bin/ld: src.c:(.text+0x58): undefined reference to `pthread_cancel'
/usr/bin/ld: src.c:(.text+0x68): undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status
gmake[2]: *** [CMakeFiles/cmTC_a0dd7.dir/build.make:106: cmTC_a0dd7] Error 1
gmake[2]: Leaving directory '/home/pi/mjpg-streamer/mjpg-streamer-experimental/_build/CMakeFiles/CMakeTmp'
gmake[1]: *** [Makefile:140: cmTC_a0dd7/fast] Error 2
gmake[1]: Leaving directory '/home/pi/mjpg-streamer/mjpg-streamer-experimental/_build/CMakeFiles/CMakeTmp'


Source file was:
#include <pthread.h>

void* test_func(void* data)
{
  return data;
}

int main(void)
{
  pthread_t thread;
  pthread_create(&thread, NULL, test_func, NULL);
  pthread_detach(thread);
  pthread_cancel(thread);
  pthread_join(thread, NULL);
  pthread_atfork(NULL, NULL, NULL);
  pthread_exit(NULL);

  return 0;
}

Determining if the function pthread_create exists in the pthreads failed with the following output:
Change Dir: /home/pi/mjpg-streamer/mjpg-streamer-experimental/_build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/gmake cmTC_35e26/fast && gmake[1]: Entering directory '/home/pi/mjpg-streamer/mjpg-streamer-experimental/_build/CMakeFiles/CMakeTmp'
/usr/bin/gmake  -f CMakeFiles/cmTC_35e26.dir/build.make CMakeFiles/cmTC_35e26.dir/build
gmake[2]: Entering directory '/home/pi/mjpg-streamer/mjpg-streamer-experimental/_build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_35e26.dir/CheckFunctionExists.c.o
/usr/bin/cc   -DCHECK_FUNCTION_EXISTS=pthread_create -o CMakeFiles/cmTC_35e26.dir/CheckFunctionExists.c.o -c /usr/share/cmake-3.18/Modules/CheckFunctionExists.c
Linking C executable cmTC_35e26
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_35e26.dir/link.txt --verbose=1
/usr/bin/cc  -DCHECK_FUNCTION_EXISTS=pthread_create -rdynamic CMakeFiles/cmTC_35e26.dir/CheckFunctionExists.c.o -o cmTC_35e26  -lpthreads 
/usr/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
gmake[2]: *** [CMakeFiles/cmTC_35e26.dir/build.make:106: cmTC_35e26] Error 1
gmake[2]: Leaving directory '/home/pi/mjpg-streamer/mjpg-streamer-experimental/_build/CMakeFiles/CMakeTmp'
gmake[1]: *** [Makefile:140: cmTC_35e26/fast] Error 2
gmake[1]: Leaving directory '/home/pi/mjpg-streamer/mjpg-streamer-experimental/_build/CMakeFiles/CMakeTmp'