Performing C SOURCE FILE Test CMAKE_HAVE_LIBC_PTHREAD failed with the following output:
Change Dir: /tmp/instance-1/output-1/build/cegui-00b4e1fe174da53b7ed726ab5970ba51bd5b5ee0/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_66a97/fast && make[1]: Entering directory '/tmp/instance-1/output-1/build/cegui-00b4e1fe174da53b7ed726ab5970ba51bd5b5ee0/CMakeFiles/CMakeTmp'
/usr/bin/make  -f CMakeFiles/cmTC_66a97.dir/build.make CMakeFiles/cmTC_66a97.dir/build
make[2]: Entering directory '/tmp/instance-1/output-1/build/cegui-00b4e1fe174da53b7ed726ab5970ba51bd5b5ee0/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_66a97.dir/src.c.o
/tmp/instance-1/output-1/host/bin/x86_64-linux-gcc --sysroot=/tmp/instance-1/output-1/host/x86_64-buildroot-linux-gnu/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -g0 -D_FORTIFY_SOURCE=1 -std=c11 -DCMAKE_HAVE_LIBC_PTHREAD  -DNDEBUG   -o CMakeFiles/cmTC_66a97.dir/src.c.o   -c /tmp/instance-1/output-1/build/cegui-00b4e1fe174da53b7ed726ab5970ba51bd5b5ee0/CMakeFiles/CMakeTmp/src.c
Linking C executable cmTC_66a97
/tmp/instance-1/output-1/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_66a97.dir/link.txt --verbose=1
/tmp/instance-1/output-1/host/bin/x86_64-linux-gcc --sysroot=/tmp/instance-1/output-1/host/x86_64-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -g0 -D_FORTIFY_SOURCE=1 -std=c11 -DCMAKE_HAVE_LIBC_PTHREAD  -DNDEBUG    CMakeFiles/cmTC_66a97.dir/src.c.o  -o cmTC_66a97 
/tmp/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/x86_64-buildroot-linux-gnu/10.2.0/../../../../x86_64-buildroot-linux-gnu/bin/ld: CMakeFiles/cmTC_66a97.dir/src.c.o: in function `main':
src.c:(.text.startup+0x23): undefined reference to `pthread_create'
/tmp/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/x86_64-buildroot-linux-gnu/10.2.0/../../../../x86_64-buildroot-linux-gnu/bin/ld: src.c:(.text.startup+0x2c): undefined reference to `pthread_detach'
/tmp/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/x86_64-buildroot-linux-gnu/10.2.0/../../../../x86_64-buildroot-linux-gnu/bin/ld: src.c:(.text.startup+0x37): undefined reference to `pthread_join'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/cmTC_66a97.dir/build.make:90: cmTC_66a97] Error 1
make[2]: Leaving directory '/tmp/instance-1/output-1/build/cegui-00b4e1fe174da53b7ed726ab5970ba51bd5b5ee0/CMakeFiles/CMakeTmp'
make[1]: *** [Makefile:124: cmTC_66a97/fast] Error 2
make[1]: Leaving directory '/tmp/instance-1/output-1/build/cegui-00b4e1fe174da53b7ed726ab5970ba51bd5b5ee0/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_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: /tmp/instance-1/output-1/build/cegui-00b4e1fe174da53b7ed726ab5970ba51bd5b5ee0/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_e9ea2/fast && make[1]: Entering directory '/tmp/instance-1/output-1/build/cegui-00b4e1fe174da53b7ed726ab5970ba51bd5b5ee0/CMakeFiles/CMakeTmp'
/usr/bin/make  -f CMakeFiles/cmTC_e9ea2.dir/build.make CMakeFiles/cmTC_e9ea2.dir/build
make[2]: Entering directory '/tmp/instance-1/output-1/build/cegui-00b4e1fe174da53b7ed726ab5970ba51bd5b5ee0/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_e9ea2.dir/CheckFunctionExists.c.o
/tmp/instance-1/output-1/host/bin/x86_64-linux-gcc --sysroot=/tmp/instance-1/output-1/host/x86_64-buildroot-linux-gnu/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -g0 -D_FORTIFY_SOURCE=1 -std=c11 -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG   -o CMakeFiles/cmTC_e9ea2.dir/CheckFunctionExists.c.o   -c /tmp/instance-1/output-1/host/share/cmake-3.16/Modules/CheckFunctionExists.c
Linking C executable cmTC_e9ea2
/tmp/instance-1/output-1/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_e9ea2.dir/link.txt --verbose=1
/tmp/instance-1/output-1/host/bin/x86_64-linux-gcc --sysroot=/tmp/instance-1/output-1/host/x86_64-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -g0 -D_FORTIFY_SOURCE=1 -std=c11 -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG    CMakeFiles/cmTC_e9ea2.dir/CheckFunctionExists.c.o  -o cmTC_e9ea2  -lpthreads 
/tmp/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/x86_64-buildroot-linux-gnu/10.2.0/../../../../x86_64-buildroot-linux-gnu/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/cmTC_e9ea2.dir/build.make:90: cmTC_e9ea2] Error 1
make[2]: Leaving directory '/tmp/instance-1/output-1/build/cegui-00b4e1fe174da53b7ed726ab5970ba51bd5b5ee0/CMakeFiles/CMakeTmp'
make[1]: *** [Makefile:124: cmTC_e9ea2/fast] Error 2
make[1]: Leaving directory '/tmp/instance-1/output-1/build/cegui-00b4e1fe174da53b7ed726ab5970ba51bd5b5ee0/CMakeFiles/CMakeTmp'