Determining if the pthread_create exist failed with the following output:
Change Dir: /srv/storage/autobuild/run/instance-0/output-1/build/libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_4652a/fast"
make[1]: Entering directory '/srv/storage/autobuild/run/instance-0/output-1/build/libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_4652a.dir/build.make CMakeFiles/cmTC_4652a.dir/build
make[2]: Entering directory '/srv/storage/autobuild/run/instance-0/output-1/build/libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'
Building CXX object CMakeFiles/cmTC_4652a.dir/CheckSymbolExists.cxx.o
/srv/storage/autobuild/run/instance-0/output-1/host/bin/mips-linux-gnu-g++ --sysroot=/srv/storage/autobuild/run/instance-0/output-1/host/mips64el-buildroot-linux-gnu/sysroot    -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG   -o CMakeFiles/cmTC_4652a.dir/CheckSymbolExists.cxx.o -c /srv/storage/autobuild/run/instance-0/output-1/build/libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp/CheckSymbolExists.cxx
Linking CXX executable cmTC_4652a
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_4652a.dir/link.txt --verbose=1
/srv/storage/autobuild/run/instance-0/output-1/host/bin/mips-linux-gnu-g++ --sysroot=/srv/storage/autobuild/run/instance-0/output-1/host/mips64el-buildroot-linux-gnu/sysroot  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG    CMakeFiles/cmTC_4652a.dir/CheckSymbolExists.cxx.o  -o cmTC_4652a 
CMakeFiles/cmTC_4652a.dir/CheckSymbolExists.cxx.o: In function `main':
CheckSymbolExists.cxx:(.text.startup+0xc): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/cmTC_4652a.dir/build.make:87: cmTC_4652a] Error 1
make[2]: Leaving directory '/srv/storage/autobuild/run/instance-0/output-1/build/libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'
make[1]: *** [Makefile:121: cmTC_4652a/fast] Error 2
make[1]: Leaving directory '/srv/storage/autobuild/run/instance-0/output-1/build/libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'

File /srv/storage/autobuild/run/instance-0/output-1/build/libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp/CheckSymbolExists.cxx:
/* */
#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: /srv/storage/autobuild/run/instance-0/output-1/build/libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_5bcf0/fast"
make[1]: Entering directory '/srv/storage/autobuild/run/instance-0/output-1/build/libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_5bcf0.dir/build.make CMakeFiles/cmTC_5bcf0.dir/build
make[2]: Entering directory '/srv/storage/autobuild/run/instance-0/output-1/build/libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'
Building CXX object CMakeFiles/cmTC_5bcf0.dir/CheckFunctionExists.cxx.o
/srv/storage/autobuild/run/instance-0/output-1/host/bin/mips-linux-gnu-g++ --sysroot=/srv/storage/autobuild/run/instance-0/output-1/host/mips64el-buildroot-linux-gnu/sysroot    -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG   -o CMakeFiles/cmTC_5bcf0.dir/CheckFunctionExists.cxx.o -c /srv/storage/autobuild/run/instance-0/output-1/build/libabseil-cpp-20200923.2/CMakeFiles/CheckLibraryExists/CheckFunctionExists.cxx
Linking CXX executable cmTC_5bcf0
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_5bcf0.dir/link.txt --verbose=1
/srv/storage/autobuild/run/instance-0/output-1/host/bin/mips-linux-gnu-g++ --sysroot=/srv/storage/autobuild/run/instance-0/output-1/host/mips64el-buildroot-linux-gnu/sysroot  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG    CMakeFiles/cmTC_5bcf0.dir/CheckFunctionExists.cxx.o  -o cmTC_5bcf0 -lpthreads 
/srv/storage/autobuild/run/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/mips-linux-gnu/5.3.0/../../../../mips-linux-gnu/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/cmTC_5bcf0.dir/build.make:87: cmTC_5bcf0] Error 1
make[2]: Leaving directory '/srv/storage/autobuild/run/instance-0/output-1/build/libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'
make[1]: *** [Makefile:121: cmTC_5bcf0/fast] Error 2
make[1]: Leaving directory '/srv/storage/autobuild/run/instance-0/output-1/build/libabseil-cpp-20200923.2/CMakeFiles/CMakeTmp'