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

Run Build Command:"/usr/bin/make" "cmTC_c7d23/fast"
make[1]: Entering directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_c7d23.dir/build.make CMakeFiles/cmTC_c7d23.dir/build
make[2]: Entering directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_c7d23.dir/CheckSymbolExists.c.o
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/bin/nios2-linux-gcc --sysroot=/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/nios2-buildroot-linux-gnu/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG   -o CMakeFiles/cmTC_c7d23.dir/CheckSymbolExists.c.o   -c /home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_c7d23
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_c7d23.dir/link.txt --verbose=1
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/bin/nios2-linux-gcc --sysroot=/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/nios2-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os   -DNDEBUG    CMakeFiles/cmTC_c7d23.dir/CheckSymbolExists.c.o  -o cmTC_c7d23 
CMakeFiles/cmTC_c7d23.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text.startup+0x4): undefined reference to `pthread_create'
CheckSymbolExists.c:(.text.startup+0x8): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_c7d23.dir/build.make:97: recipe for target 'cmTC_c7d23' failed
make[2]: *** [cmTC_c7d23] Error 1
make[2]: Leaving directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_c7d23/fast' failed
make[1]: *** [cmTC_c7d23/fast] Error 2
make[1]: Leaving directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'

File /home/rclinux/rc-buildroot-test/scripts/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: /home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_755de/fast"
make[1]: Entering directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_755de.dir/build.make CMakeFiles/cmTC_755de.dir/build
make[2]: Entering directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_755de.dir/CheckFunctionExists.c.o
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/bin/nios2-linux-gcc --sysroot=/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/nios2-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_755de.dir/CheckFunctionExists.c.o   -c /home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/share/cmake-3.8/Modules/CheckFunctionExists.c
Linking C executable cmTC_755de
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_755de.dir/link.txt --verbose=1
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/bin/nios2-linux-gcc --sysroot=/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/nios2-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG    CMakeFiles/cmTC_755de.dir/CheckFunctionExists.c.o  -o cmTC_755de -lpthreads 
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/opt/ext-toolchain/bin/../lib/gcc/nios2-buildroot-linux-gnu/7.4.0/../../../../nios2-buildroot-linux-gnu/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_755de.dir/build.make:97: recipe for target 'cmTC_755de' failed
make[2]: *** [cmTC_755de] Error 1
make[2]: Leaving directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_755de/fast' failed
make[1]: *** [cmTC_755de/fast] Error 2
make[1]: Leaving directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/optee-client-3.4.0/CMakeFiles/CMakeTmp'