Determining if the pthread_create exist failed with the following output:
Change Dir: /data/buildroot/buildroot-test/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_11891/fast"
make[1]: Entering directory '/data/buildroot/buildroot-test/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_11891.dir/build.make CMakeFiles/cmTC_11891.dir/build
make[2]: Entering directory '/data/buildroot/buildroot-test/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_11891.dir/CheckSymbolExists.c.o
/data/buildroot/buildroot-test/instance-1/output/host/bin/arm-none-linux-gnueabi-gcc --sysroot=/data/buildroot/buildroot-test/instance-1/output/host/arm-buildroot-linux-gnueabi/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG   -o CMakeFiles/cmTC_11891.dir/CheckSymbolExists.c.o   -c /data/buildroot/buildroot-test/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_11891
/data/buildroot/buildroot-test/instance-1/output/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_11891.dir/link.txt --verbose=1
/data/buildroot/buildroot-test/instance-1/output/host/bin/arm-none-linux-gnueabi-gcc --sysroot=/data/buildroot/buildroot-test/instance-1/output/host/arm-buildroot-linux-gnueabi/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG    CMakeFiles/cmTC_11891.dir/CheckSymbolExists.c.o  -o cmTC_11891 
CMakeFiles/cmTC_11891.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text.startup+0x8): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/cmTC_11891.dir/build.make:98: cmTC_11891] Error 1
make[2]: Leaving directory '/data/buildroot/buildroot-test/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
make[1]: *** [Makefile:126: cmTC_11891/fast] Error 2
make[1]: Leaving directory '/data/buildroot/buildroot-test/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'

File /data/buildroot/buildroot-test/instance-1/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: /data/buildroot/buildroot-test/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_71477/fast"
make[1]: Entering directory '/data/buildroot/buildroot-test/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_71477.dir/build.make CMakeFiles/cmTC_71477.dir/build
make[2]: Entering directory '/data/buildroot/buildroot-test/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_71477.dir/CheckFunctionExists.c.o
/data/buildroot/buildroot-test/instance-1/output/host/bin/arm-none-linux-gnueabi-gcc --sysroot=/data/buildroot/buildroot-test/instance-1/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_71477.dir/CheckFunctionExists.c.o   -c /data/buildroot/buildroot-test/instance-1/output/host/share/cmake-3.8/Modules/CheckFunctionExists.c
Linking C executable cmTC_71477
/data/buildroot/buildroot-test/instance-1/output/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_71477.dir/link.txt --verbose=1
/data/buildroot/buildroot-test/instance-1/output/host/bin/arm-none-linux-gnueabi-gcc --sysroot=/data/buildroot/buildroot-test/instance-1/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_71477.dir/CheckFunctionExists.c.o  -o cmTC_71477 -lpthreads 
/data/buildroot/buildroot-test/instance-1/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
make[2]: *** [CMakeFiles/cmTC_71477.dir/build.make:98: cmTC_71477] Error 1
make[2]: Leaving directory '/data/buildroot/buildroot-test/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
make[1]: *** [Makefile:126: cmTC_71477/fast] Error 2
make[1]: Leaving directory '/data/buildroot/buildroot-test/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'