Determining if the pthread_create exist failed with the following output:
Change Dir: /home/buildroot/buildroot-test/instance-1/output/build/libiio-0.14/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_3ab5a/fast"
make[1]: Entering directory `/home/buildroot/buildroot-test/instance-1/output/build/libiio-0.14/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_3ab5a.dir/build.make CMakeFiles/cmTC_3ab5a.dir/build
make[2]: Entering directory `/home/buildroot/buildroot-test/instance-1/output/build/libiio-0.14/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_3ab5a.dir/CheckSymbolExists.c.o
/home/buildroot/buildroot-test/instance-1/output/host/bin/arc-buildroot-linux-gnu-gcc --sysroot=/home/buildroot/buildroot-test/instance-1/output/host/arc-buildroot-linux-gnu/sysroot -D_GNU_SOURCE=1  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -matomic -Os -fvisibility=hidden -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -Wpedantic   -DNDEBUG   -o CMakeFiles/cmTC_3ab5a.dir/CheckSymbolExists.c.o   -c /home/buildroot/buildroot-test/instance-1/output/build/libiio-0.14/CMakeFiles/CMakeTmp/CheckSymbolExists.c
/home/buildroot/buildroot-test/instance-1/output/build/libiio-0.14/CMakeFiles/CMakeTmp/CheckSymbolExists.c: In function 'main':
/home/buildroot/buildroot-test/instance-1/output/build/libiio-0.14/CMakeFiles/CMakeTmp/CheckSymbolExists.c:8:11: warning: ISO C forbids conversion of function pointer to object pointer type [-Wpedantic]
   return ((int*)(&pthread_create))[argc];
           ^
Linking C executable cmTC_3ab5a
/home/buildroot/buildroot-test/instance-1/output/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_3ab5a.dir/link.txt --verbose=1
/home/buildroot/buildroot-test/instance-1/output/host/bin/arc-buildroot-linux-gnu-gcc --sysroot=/home/buildroot/buildroot-test/instance-1/output/host/arc-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -matomic -Os -fvisibility=hidden -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -Wpedantic   -DNDEBUG    -rdynamic CMakeFiles/cmTC_3ab5a.dir/CheckSymbolExists.c.o  -o cmTC_3ab5a 
CMakeFiles/cmTC_3ab5a.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text.startup+0x4): undefined reference to `pthread_create'
CheckSymbolExists.c:(.text.startup+0x4): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
make[2]: *** [cmTC_3ab5a] Error 1
make[2]: Leaving directory `/home/buildroot/buildroot-test/instance-1/output/build/libiio-0.14/CMakeFiles/CMakeTmp'
make[1]: *** [cmTC_3ab5a/fast] Error 2
make[1]: Leaving directory `/home/buildroot/buildroot-test/instance-1/output/build/libiio-0.14/CMakeFiles/CMakeTmp'

File /home/buildroot/buildroot-test/instance-1/output/build/libiio-0.14/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/buildroot-test/instance-1/output/build/libiio-0.14/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_dd863/fast"
make[1]: Entering directory `/home/buildroot/buildroot-test/instance-1/output/build/libiio-0.14/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_dd863.dir/build.make CMakeFiles/cmTC_dd863.dir/build
make[2]: Entering directory `/home/buildroot/buildroot-test/instance-1/output/build/libiio-0.14/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_dd863.dir/CheckFunctionExists.c.o
/home/buildroot/buildroot-test/instance-1/output/host/bin/arc-buildroot-linux-gnu-gcc --sysroot=/home/buildroot/buildroot-test/instance-1/output/host/arc-buildroot-linux-gnu/sysroot -D_GNU_SOURCE=1  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -matomic -Os -fvisibility=hidden -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -Wpedantic -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG   -o CMakeFiles/cmTC_dd863.dir/CheckFunctionExists.c.o   -c /home/buildroot/buildroot-test/instance-1/output/host/share/cmake-3.8/Modules/CheckFunctionExists.c
Linking C executable cmTC_dd863
/home/buildroot/buildroot-test/instance-1/output/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_dd863.dir/link.txt --verbose=1
/home/buildroot/buildroot-test/instance-1/output/host/bin/arc-buildroot-linux-gnu-gcc --sysroot=/home/buildroot/buildroot-test/instance-1/output/host/arc-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -matomic -Os -fvisibility=hidden -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -Wpedantic -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG    -rdynamic CMakeFiles/cmTC_dd863.dir/CheckFunctionExists.c.o  -o cmTC_dd863 -lpthreads 
/home/buildroot/buildroot-test/instance-1/output/host/lib/gcc/arc-buildroot-linux-gnu/7.3.1/../../../../arc-buildroot-linux-gnu/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
make[2]: *** [cmTC_dd863] Error 1
make[2]: Leaving directory `/home/buildroot/buildroot-test/instance-1/output/build/libiio-0.14/CMakeFiles/CMakeTmp'
make[1]: *** [cmTC_dd863/fast] Error 2
make[1]: Leaving directory `/home/buildroot/buildroot-test/instance-1/output/build/libiio-0.14/CMakeFiles/CMakeTmp'