Need help linking to boost thread library

uname -a
Linux Dell-Ubuntu 4.15.0-66-generic #75-Ubuntu SMP Tue Oct 1 05:24:09 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux

I am trying to compile and link a very simple program named "testthread.cpp", merely intended to test my boost installation:

--------------------------------------------------------
#include <boost/thread.hpp>
#include <iostream>

int main()
{
  std::cout << boost::thread::hardware_concurrency() << std::endl;
}
-----------------------------------------------------------

When I compile with the following command:
gcc -o testthread testthread.cpp -lboost_thread

I get the following errors:
/usr/bin/ld: /tmp/ccsW57B3.o: undefined reference to symbol '_ZN5boost6system15system_categoryEv'
//usr/lib/x86_64-linux-gnu/libboost_system.so.1.65.1: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

Boost was installed as follows:

sudo apt-get install libboost-all-dev

What am I doing wrong?

Thanks,
Allan