Link dynamic_load-example against -pthread

[RValner]

I was trying to test the dynamic_load-example with jaeger tracer in libjaegertracing.so and ended up with a segmentation fault. When I tested dynamic_load-example with libopentracing_mocktracer.so then everything worked as expected.

gdb indicated, that the segmentation fault occurs right when libjaegertracing.so tries to create a std::thread that is related to reporting the traces. The problem is solved if I link the dynamic_load-example against -pthread in the CMakeLists.txt:

target_link_libraries(dynamic_load-example opentracing -pthread)

This is likely a tracing library specific problem but it took a significant amount of time to realize the underlying issue, so would it be reasonable to add -pthread to the CMakeLists.txt
when building on unix?