Determining if the pthread_create exist failed with the following output:
Change Dir: /home/buildroot/build/instance-1/output-1/build/libiio-0.18/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_ebb52/fast"
make[1]: Entering directory '/home/buildroot/build/instance-1/output-1/build/libiio-0.18/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_ebb52.dir/build.make CMakeFiles/cmTC_ebb52.dir/build
make[2]: Entering directory '/home/buildroot/build/instance-1/output-1/build/libiio-0.18/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_ebb52.dir/CheckSymbolExists.c.o
/home/buildroot/build/instance-1/output-1/host/bin/arm-buildroot-linux-gnueabihf-gcc --sysroot=/home/buildroot/build/instance-1/output-1/host/arm-buildroot-linux-gnueabihf/sysroot -D_GNU_SOURCE=1  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -fvisibility=hidden -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -Wpedantic   -DNDEBUG   -o CMakeFiles/cmTC_ebb52.dir/CheckSymbolExists.c.o   -c /home/buildroot/build/instance-1/output-1/build/libiio-0.18/CMakeFiles/CMakeTmp/CheckSymbolExists.c
/home/buildroot/build/instance-1/output-1/build/libiio-0.18/CMakeFiles/CMakeTmp/CheckSymbolExists.c: In function 'main':
/home/buildroot/build/instance-1/output-1/build/libiio-0.18/CMakeFiles/CMakeTmp/CheckSymbolExists.c:8:11: warning: ISO C forbids conversion of function pointer to object pointer type [-Wpedantic]
    8 |   return ((int*)(&pthread_create))[argc];
      |           ^
Linking C executable cmTC_ebb52
/home/buildroot/build/instance-1/output-1/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_ebb52.dir/link.txt --verbose=1
/home/buildroot/build/instance-1/output-1/host/bin/arm-buildroot-linux-gnueabihf-gcc --sysroot=/home/buildroot/build/instance-1/output-1/host/arm-buildroot-linux-gnueabihf/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -fvisibility=hidden -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -Wpedantic   -DNDEBUG    -rdynamic CMakeFiles/cmTC_ebb52.dir/CheckSymbolExists.c.o  -o cmTC_ebb52 
/home/buildroot/build/instance-1/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/9.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: CMakeFiles/cmTC_ebb52.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_ebb52.dir/build.make:97: recipe for target 'cmTC_ebb52' failed
make[2]: *** [cmTC_ebb52] Error 1
make[2]: Leaving directory '/home/buildroot/build/instance-1/output-1/build/libiio-0.18/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_ebb52/fast' failed
make[1]: *** [cmTC_ebb52/fast] Error 2
make[1]: Leaving directory '/home/buildroot/build/instance-1/output-1/build/libiio-0.18/CMakeFiles/CMakeTmp'

File /home/buildroot/build/instance-1/output-1/build/libiio-0.18/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-1/output-1/build/libiio-0.18/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_bbdd6/fast"
make[1]: Entering directory '/home/buildroot/build/instance-1/output-1/build/libiio-0.18/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_bbdd6.dir/build.make CMakeFiles/cmTC_bbdd6.dir/build
make[2]: Entering directory '/home/buildroot/build/instance-1/output-1/build/libiio-0.18/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_bbdd6.dir/CheckFunctionExists.c.o
/home/buildroot/build/instance-1/output-1/host/bin/arm-buildroot-linux-gnueabihf-gcc --sysroot=/home/buildroot/build/instance-1/output-1/host/arm-buildroot-linux-gnueabihf/sysroot -D_GNU_SOURCE=1  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -fvisibility=hidden -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -Wpedantic -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG   -o CMakeFiles/cmTC_bbdd6.dir/CheckFunctionExists.c.o   -c /home/buildroot/build/instance-1/output-1/host/share/cmake-3.8/Modules/CheckFunctionExists.c
Linking C executable cmTC_bbdd6
/home/buildroot/build/instance-1/output-1/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_bbdd6.dir/link.txt --verbose=1
/home/buildroot/build/instance-1/output-1/host/bin/arm-buildroot-linux-gnueabihf-gcc --sysroot=/home/buildroot/build/instance-1/output-1/host/arm-buildroot-linux-gnueabihf/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -fvisibility=hidden -Wall -Wextra -Wno-unused-parameter -Wno-sign-compare -Wpedantic -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG    -rdynamic CMakeFiles/cmTC_bbdd6.dir/CheckFunctionExists.c.o  -o cmTC_bbdd6 -lpthreads 
/home/buildroot/build/instance-1/output-1/host/lib/gcc/arm-buildroot-linux-gnueabihf/9.2.0/../../../../arm-buildroot-linux-gnueabihf/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_bbdd6.dir/build.make:97: recipe for target 'cmTC_bbdd6' failed
make[2]: *** [cmTC_bbdd6] Error 1
make[2]: Leaving directory '/home/buildroot/build/instance-1/output-1/build/libiio-0.18/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_bbdd6/fast' failed
make[1]: *** [cmTC_bbdd6/fast] Error 2
make[1]: Leaving directory '/home/buildroot/build/instance-1/output-1/build/libiio-0.18/CMakeFiles/CMakeTmp'