Performing C SOURCE FILE Test CMAKE_HAVE_LIBC_PTHREAD failed with the following output:
Change Dir: /var/tmp/portage/media-gfx/bonzomatic-2021.03.07/work/bonzomatic-2021.03.07_build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/ninja cmTC_c6708 && [1/2] Building C object CMakeFiles/cmTC_c6708.dir/src.c.o
[2/2] Linking C executable cmTC_c6708
FAILED: cmTC_c6708 
: && /usr/bin/x86_64-pc-linux-gnu-gcc -O2 -pipe -march=x86-64 -frecord-gcc-switches -fno-diagnostics-color -Wl,-O1 -Wl,--as-needed -Wl,--defsym=__gentoo_check_ldflags__=0   -rdynamic CMakeFiles/cmTC_c6708.dir/src.c.o -o cmTC_c6708   && :
/usr/lib/gcc/x86_64-pc-linux-gnu/10.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: CMakeFiles/cmTC_c6708.dir/src.c.o: in function `main':
src.c:(.text.startup+0x23): undefined reference to `pthread_create'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: src.c:(.text.startup+0x2c): undefined reference to `pthread_detach'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: src.c:(.text.startup+0x35): undefined reference to `pthread_cancel'
/usr/lib/gcc/x86_64-pc-linux-gnu/10.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: src.c:(.text.startup+0x40): undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.


Source file was:
#include <pthread.h>

static 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: /var/tmp/portage/media-gfx/bonzomatic-2021.03.07/work/bonzomatic-2021.03.07_build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/ninja cmTC_99499 && [1/2] Building C object CMakeFiles/cmTC_99499.dir/CheckFunctionExists.c.o
[2/2] Linking C executable cmTC_99499
FAILED: cmTC_99499 
: && /usr/bin/x86_64-pc-linux-gnu-gcc -O2 -pipe -march=x86-64 -frecord-gcc-switches -fno-diagnostics-color -DCHECK_FUNCTION_EXISTS=pthread_create -Wl,-O1 -Wl,--as-needed -Wl,--defsym=__gentoo_check_ldflags__=0   -rdynamic CMakeFiles/cmTC_99499.dir/CheckFunctionExists.c.o -o cmTC_99499  -lpthreads && :
/usr/lib/gcc/x86_64-pc-linux-gnu/10.3.0/../../../../x86_64-pc-linux-gnu/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
ninja: build stopped: subcommand failed.