Performing C++ SOURCE FILE Test CMAKE_HAVE_LIBC_PTHREAD failed with the following output:
Change Dir: /home/buildroot/autobuild/run/instance-1/output-1/build/host-libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_51d8f/fast && make[1]: Entering directory `/home/buildroot/autobuild/run/instance-1/output-1/build/host-libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_51d8f.dir/build.make CMakeFiles/cmTC_51d8f.dir/build
make[2]: Entering directory `/home/buildroot/autobuild/run/instance-1/output-1/build/host-libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'
Building CXX object CMakeFiles/cmTC_51d8f.dir/src.cxx.o
/usr/bin/g++    -O2 -I/home/buildroot/autobuild/run/instance-1/output-1/host/include -DCMAKE_HAVE_LIBC_PTHREAD   -o CMakeFiles/cmTC_51d8f.dir/src.cxx.o -c /home/buildroot/autobuild/run/instance-1/output-1/build/host-libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp/src.cxx
Linking CXX executable cmTC_51d8f
/home/buildroot/autobuild/run/instance-1/output-1/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_51d8f.dir/link.txt --verbose=1
/usr/bin/g++  -O2 -I/home/buildroot/autobuild/run/instance-1/output-1/host/include -DCMAKE_HAVE_LIBC_PTHREAD  -L/home/buildroot/autobuild/run/instance-1/output-1/host/lib -Wl,-rpath,/home/buildroot/autobuild/run/instance-1/output-1/host/lib  CMakeFiles/cmTC_51d8f.dir/src.cxx.o  -o cmTC_51d8f 
CMakeFiles/cmTC_51d8f.dir/src.cxx.o: In function `main':
src.cxx:(.text.startup+0x13): undefined reference to `pthread_create'
src.cxx:(.text.startup+0x1d): undefined reference to `pthread_detach'
src.cxx:(.text.startup+0x29): undefined reference to `pthread_join'
src.cxx:(.text.startup+0x34): undefined reference to `pthread_atfork'
collect2: error: ld returned 1 exit status
make[2]: *** [cmTC_51d8f] Error 1
make[2]: Leaving directory `/home/buildroot/autobuild/run/instance-1/output-1/build/host-libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'
make[1]: *** [cmTC_51d8f/fast] Error 2
make[1]: Leaving directory `/home/buildroot/autobuild/run/instance-1/output-1/build/host-libabseil-cpp-20200923.2/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: /home/buildroot/autobuild/run/instance-1/output-1/build/host-libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_8ed64/fast && make[1]: Entering directory `/home/buildroot/autobuild/run/instance-1/output-1/build/host-libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_8ed64.dir/build.make CMakeFiles/cmTC_8ed64.dir/build
make[2]: Entering directory `/home/buildroot/autobuild/run/instance-1/output-1/build/host-libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'
Building CXX object CMakeFiles/cmTC_8ed64.dir/CheckFunctionExists.cxx.o
/usr/bin/g++    -O2 -I/home/buildroot/autobuild/run/instance-1/output-1/host/include -DCHECK_FUNCTION_EXISTS=pthread_create   -o CMakeFiles/cmTC_8ed64.dir/CheckFunctionExists.cxx.o -c /home/buildroot/autobuild/run/instance-1/output-1/build/host-libabseil-cpp-20200923.2/CMakeFiles/CheckLibraryExists/CheckFunctionExists.cxx
Linking CXX executable cmTC_8ed64
/home/buildroot/autobuild/run/instance-1/output-1/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_8ed64.dir/link.txt --verbose=1
/usr/bin/g++  -O2 -I/home/buildroot/autobuild/run/instance-1/output-1/host/include -DCHECK_FUNCTION_EXISTS=pthread_create  -L/home/buildroot/autobuild/run/instance-1/output-1/host/lib -Wl,-rpath,/home/buildroot/autobuild/run/instance-1/output-1/host/lib  CMakeFiles/cmTC_8ed64.dir/CheckFunctionExists.cxx.o  -o cmTC_8ed64 -lpthreads 
/usr/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
make[2]: *** [cmTC_8ed64] Error 1
make[2]: Leaving directory `/home/buildroot/autobuild/run/instance-1/output-1/build/host-libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'
make[1]: *** [cmTC_8ed64/fast] Error 2
make[1]: Leaving directory `/home/buildroot/autobuild/run/instance-1/output-1/build/host-libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'