Determining if the pthread_create exist failed with the following output:
Change Dir: /home/buildroot/build/instance-0/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_e5c23/fast"
make[1]: Entering directory '/home/buildroot/build/instance-0/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_e5c23.dir/build.make CMakeFiles/cmTC_e5c23.dir/build
make[2]: Entering directory '/home/buildroot/build/instance-0/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_e5c23.dir/CheckSymbolExists.c.o
/home/buildroot/build/instance-0/output/host/bin/arm-none-linux-gnueabi-gcc --sysroot=/home/buildroot/build/instance-0/output/host/arm-buildroot-linux-gnueabi/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG   -o CMakeFiles/cmTC_e5c23.dir/CheckSymbolExists.c.o   -c /home/buildroot/build/instance-0/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_e5c23
/home/buildroot/build/instance-0/output/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_e5c23.dir/link.txt --verbose=1
/home/buildroot/build/instance-0/output/host/bin/arm-none-linux-gnueabi-gcc --sysroot=/home/buildroot/build/instance-0/output/host/arm-buildroot-linux-gnueabi/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG    CMakeFiles/cmTC_e5c23.dir/CheckSymbolExists.c.o  -o cmTC_e5c23 
CMakeFiles/cmTC_e5c23.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text.startup+0xc): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_e5c23.dir/build.make:97: recipe for target 'cmTC_e5c23' failed
make[2]: *** [cmTC_e5c23] Error 1
make[2]: Leaving directory '/home/buildroot/build/instance-0/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_e5c23/fast' failed
make[1]: *** [cmTC_e5c23/fast] Error 2
make[1]: Leaving directory '/home/buildroot/build/instance-0/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'

File /home/buildroot/build/instance-0/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp/CheckSymbolExists.c:
/* */
#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: /home/buildroot/build/instance-0/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_fa9cc/fast"
make[1]: Entering directory '/home/buildroot/build/instance-0/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_fa9cc.dir/build.make CMakeFiles/cmTC_fa9cc.dir/build
make[2]: Entering directory '/home/buildroot/build/instance-0/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_fa9cc.dir/CheckFunctionExists.c.o
/home/buildroot/build/instance-0/output/host/bin/arm-none-linux-gnueabi-gcc --sysroot=/home/buildroot/build/instance-0/output/host/arm-buildroot-linux-gnueabi/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG   -o CMakeFiles/cmTC_fa9cc.dir/CheckFunctionExists.c.o   -c /home/buildroot/build/instance-0/output/host/share/cmake-3.8/Modules/CheckFunctionExists.c
Linking C executable cmTC_fa9cc
/home/buildroot/build/instance-0/output/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_fa9cc.dir/link.txt --verbose=1
/home/buildroot/build/instance-0/output/host/bin/arm-none-linux-gnueabi-gcc --sysroot=/home/buildroot/build/instance-0/output/host/arm-buildroot-linux-gnueabi/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG    CMakeFiles/cmTC_fa9cc.dir/CheckFunctionExists.c.o  -o cmTC_fa9cc -lpthreads 
/home/buildroot/build/instance-0/output/host/opt/ext-toolchain/bin/../lib/gcc/arm-none-linux-gnueabi/4.8.3/../../../../arm-none-linux-gnueabi/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_fa9cc.dir/build.make:97: recipe for target 'cmTC_fa9cc' failed
make[2]: *** [cmTC_fa9cc] Error 1
make[2]: Leaving directory '/home/buildroot/build/instance-0/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_fa9cc/fast' failed
make[1]: *** [cmTC_fa9cc/fast] Error 2
make[1]: Leaving directory '/home/buildroot/build/instance-0/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'