Determining if the pthread_create exist failed with the following output:
Change Dir: /home/mark/buildroot-test/instance-0/output/build/domoticz-0f411f781ae4fb4a82f592d38a3f40578c149533/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_1f34b/fast"
make[1]: Entering directory '/home/mark/buildroot-test/instance-0/output/build/domoticz-0f411f781ae4fb4a82f592d38a3f40578c149533/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_1f34b.dir/build.make CMakeFiles/cmTC_1f34b.dir/build
make[2]: Entering directory '/home/mark/buildroot-test/instance-0/output/build/domoticz-0f411f781ae4fb4a82f592d38a3f40578c149533/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_1f34b.dir/CheckSymbolExists.c.o
/home/mark/buildroot-test/instance-0/output/host/bin/riscv64-linux-gcc --sysroot=/home/mark/buildroot-test/instance-0/output/host/riscv64-buildroot-linux-gnu/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -D_FORTIFY_SOURCE=2   -DNDEBUG   -o CMakeFiles/cmTC_1f34b.dir/CheckSymbolExists.c.o   -c /home/mark/buildroot-test/instance-0/output/build/domoticz-0f411f781ae4fb4a82f592d38a3f40578c149533/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_1f34b
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_1f34b.dir/link.txt --verbose=1
/home/mark/buildroot-test/instance-0/output/host/bin/riscv64-linux-gcc --sysroot=/home/mark/buildroot-test/instance-0/output/host/riscv64-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -D_FORTIFY_SOURCE=2   -DNDEBUG    CMakeFiles/cmTC_1f34b.dir/CheckSymbolExists.c.o  -o cmTC_1f34b 
/home/mark/buildroot-test/instance-0/output/host/opt/ext-toolchain/bin/../lib/gcc/riscv64-buildroot-linux-gnu/7.4.0/../../../../riscv64-buildroot-linux-gnu/bin/ld: CMakeFiles/cmTC_1f34b.dir/CheckSymbolExists.c.o: in function `main':
CheckSymbolExists.c:(.text.startup+0x0): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/cmTC_1f34b.dir/build.make:87: cmTC_1f34b] Error 1
make[2]: Leaving directory '/home/mark/buildroot-test/instance-0/output/build/domoticz-0f411f781ae4fb4a82f592d38a3f40578c149533/CMakeFiles/CMakeTmp'
make[1]: *** [Makefile:121: cmTC_1f34b/fast] Error 2
make[1]: Leaving directory '/home/mark/buildroot-test/instance-0/output/build/domoticz-0f411f781ae4fb4a82f592d38a3f40578c149533/CMakeFiles/CMakeTmp'

File /home/mark/buildroot-test/instance-0/output/build/domoticz-0f411f781ae4fb4a82f592d38a3f40578c149533/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/mark/buildroot-test/instance-0/output/build/domoticz-0f411f781ae4fb4a82f592d38a3f40578c149533/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_f135d/fast"
make[1]: Entering directory '/home/mark/buildroot-test/instance-0/output/build/domoticz-0f411f781ae4fb4a82f592d38a3f40578c149533/CMakeFiles/CMakeTmp'
/usr/bin/make -f CMakeFiles/cmTC_f135d.dir/build.make CMakeFiles/cmTC_f135d.dir/build
make[2]: Entering directory '/home/mark/buildroot-test/instance-0/output/build/domoticz-0f411f781ae4fb4a82f592d38a3f40578c149533/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_f135d.dir/CheckFunctionExists.c.o
/home/mark/buildroot-test/instance-0/output/host/bin/riscv64-linux-gcc --sysroot=/home/mark/buildroot-test/instance-0/output/host/riscv64-buildroot-linux-gnu/sysroot   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -D_FORTIFY_SOURCE=2 -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG   -o CMakeFiles/cmTC_f135d.dir/CheckFunctionExists.c.o   -c /usr/share/cmake-3.12/Modules/CheckFunctionExists.c
Linking C executable cmTC_f135d
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_f135d.dir/link.txt --verbose=1
/home/mark/buildroot-test/instance-0/output/host/bin/riscv64-linux-gcc --sysroot=/home/mark/buildroot-test/instance-0/output/host/riscv64-buildroot-linux-gnu/sysroot -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -D_FORTIFY_SOURCE=2 -DCHECK_FUNCTION_EXISTS=pthread_create  -DNDEBUG    CMakeFiles/cmTC_f135d.dir/CheckFunctionExists.c.o  -o cmTC_f135d -lpthreads 
/home/mark/buildroot-test/instance-0/output/host/opt/ext-toolchain/bin/../lib/gcc/riscv64-buildroot-linux-gnu/7.4.0/../../../../riscv64-buildroot-linux-gnu/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
make[2]: *** [CMakeFiles/cmTC_f135d.dir/build.make:87: cmTC_f135d] Error 1
make[2]: Leaving directory '/home/mark/buildroot-test/instance-0/output/build/domoticz-0f411f781ae4fb4a82f592d38a3f40578c149533/CMakeFiles/CMakeTmp'
make[1]: *** [Makefile:121: cmTC_f135d/fast] Error 2
make[1]: Leaving directory '/home/mark/buildroot-test/instance-0/output/build/domoticz-0f411f781ae4fb4a82f592d38a3f40578c149533/CMakeFiles/CMakeTmp'