Determining if the pthread_create exist failed with the following output:
Change Dir: /home/mwo/onion-monero-blockchain-explorer/cmake-build-debug/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_55e9f/fast"
/usr/bin/make -f CMakeFiles/cmTC_55e9f.dir/build.make CMakeFiles/cmTC_55e9f.dir/build
make[1]: Entering directory '/home/mwo/onion-monero-blockchain-explorer/cmake-build-debug/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_55e9f.dir/CheckSymbolExists.c.o
/usr/bin/cc     -o CMakeFiles/cmTC_55e9f.dir/CheckSymbolExists.c.o   -c /home/mwo/onion-monero-blockchain-explorer/cmake-build-debug/CMakeFiles/CMakeTmp/CheckSymbolExists.c
Linking C executable cmTC_55e9f
/opt/clion-2016.2.2/bin/cmake/bin/cmake -E cmake_link_script CMakeFiles/cmTC_55e9f.dir/link.txt --verbose=1
/usr/bin/cc       CMakeFiles/cmTC_55e9f.dir/CheckSymbolExists.c.o  -o cmTC_55e9f -rdynamic 
CMakeFiles/cmTC_55e9f.dir/CheckSymbolExists.c.o: In function `main':
CheckSymbolExists.c:(.text+0x1b): undefined reference to `pthread_create'
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_55e9f.dir/build.make:97: recipe for target 'cmTC_55e9f' failed
make[1]: *** [cmTC_55e9f] Error 1
make[1]: Leaving directory '/home/mwo/onion-monero-blockchain-explorer/cmake-build-debug/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_55e9f/fast' failed
make: *** [cmTC_55e9f/fast] Error 2

File /home/mwo/onion-monero-blockchain-explorer/cmake-build-debug/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/mwo/onion-monero-blockchain-explorer/cmake-build-debug/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_8cda8/fast"
/usr/bin/make -f CMakeFiles/cmTC_8cda8.dir/build.make CMakeFiles/cmTC_8cda8.dir/build
make[1]: Entering directory '/home/mwo/onion-monero-blockchain-explorer/cmake-build-debug/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_8cda8.dir/CheckFunctionExists.c.o
/usr/bin/cc    -DCHECK_FUNCTION_EXISTS=pthread_create   -o CMakeFiles/cmTC_8cda8.dir/CheckFunctionExists.c.o   -c /opt/clion-2016.2.2/bin/cmake/share/cmake-3.6/Modules/CheckFunctionExists.c
Linking C executable cmTC_8cda8
/opt/clion-2016.2.2/bin/cmake/bin/cmake -E cmake_link_script CMakeFiles/cmTC_8cda8.dir/link.txt --verbose=1
/usr/bin/cc   -DCHECK_FUNCTION_EXISTS=pthread_create    CMakeFiles/cmTC_8cda8.dir/CheckFunctionExists.c.o  -o cmTC_8cda8 -rdynamic -lpthreads 
/usr/bin/ld: cannot find -lpthreads
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_8cda8.dir/build.make:97: recipe for target 'cmTC_8cda8' failed
make[1]: *** [cmTC_8cda8] Error 1
make[1]: Leaving directory '/home/mwo/onion-monero-blockchain-explorer/cmake-build-debug/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_8cda8/fast' failed
make: *** [cmTC_8cda8/fast] Error 2