Determining if the pthread_create exist failed with the following output:
Change Dir: /home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/libcpprestsdk-v2.10.8/Release/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_9ac71/fast"
make[1]: Entering directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/libcpprestsdk-v2.10.8/Release/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_9ac71.dir/build.make CMakeFiles/cmTC_9ac71.dir/build
make[2]: Entering directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/libcpprestsdk-v2.10.8/Release/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_9ac71.dir/CheckSymbolExists.c.o
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/bin/i686-ctng-linux-gnu-gcc  --sysroot=/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/i686-buildroot-linux-gnu/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -g2    -o CMakeFiles/cmTC_9ac71.dir/CheckSymbolExists.c.o   -c /home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/libcpprestsdk-v2.10.8/Release/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_9ac71
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_9ac71.dir/link.txt --verbose=1
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/bin/i686-ctng-linux-gnu-gcc  --sysroot=/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/i686-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -g2     CMakeFiles/cmTC_9ac71.dir/CheckSymbolExists.c.o  -o cmTC_9ac71 -rdynamic 
CMakeFiles/cmTC_9ac71.dir/CheckSymbolExists.c.o: In function `main':
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/libcpprestsdk-v2.10.8/Release/CMakeFiles/CMakeTmp/CheckSymbolExists.c:8: undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_9ac71.dir/build.make:97: recipe for target 'cmTC_9ac71' failed
make[2]: *** [cmTC_9ac71] Error 1
make[2]: Leaving directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/libcpprestsdk-v2.10.8/Release/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_9ac71/fast' failed
make[1]: *** [cmTC_9ac71/fast] Error 2
make[1]: Leaving directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/libcpprestsdk-v2.10.8/Release/CMakeFiles/CMakeTmp'

File /home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/libcpprestsdk-v2.10.8/Release/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/libcpprestsdk-v2.10.8/Release/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_7e85c/fast"
make[1]: Entering directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/libcpprestsdk-v2.10.8/Release/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_7e85c.dir/build.make CMakeFiles/cmTC_7e85c.dir/build
make[2]: Entering directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/libcpprestsdk-v2.10.8/Release/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_7e85c.dir/CheckFunctionExists.c.o
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/bin/i686-ctng-linux-gnu-gcc  --sysroot=/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/i686-buildroot-linux-gnu/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -g2 -DCHECK_FUNCTION_EXISTS=pthread_create   -o CMakeFiles/cmTC_7e85c.dir/CheckFunctionExists.c.o   -c /usr/share/cmake-3.5/Modules/CheckFunctionExists.c
Linking C executable cmTC_7e85c
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_7e85c.dir/link.txt --verbose=1
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/bin/i686-ctng-linux-gnu-gcc  --sysroot=/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/i686-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -g2 -DCHECK_FUNCTION_EXISTS=pthread_create    CMakeFiles/cmTC_7e85c.dir/CheckFunctionExists.c.o  -o cmTC_7e85c -rdynamic -lpthreads 
/home/rclinux/rc-buildroot-test/scripts/instance-1/output/host/opt/ext-toolchain/bin/../lib/gcc/i686-ctng-linux-gnu/4.8.2/../../../../i686-ctng-linux-gnu/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_7e85c.dir/build.make:97: recipe for target 'cmTC_7e85c' failed
make[2]: *** [cmTC_7e85c] Error 1
make[2]: Leaving directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/libcpprestsdk-v2.10.8/Release/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_7e85c/fast' failed
make[1]: *** [cmTC_7e85c/fast] Error 2
make[1]: Leaving directory '/home/rclinux/rc-buildroot-test/scripts/instance-1/output/build/libcpprestsdk-v2.10.8/Release/CMakeFiles/CMakeTmp'