message: Link to pthreads on Linux for std::mutex_shared
  C++17 CMake support isn't as magical as it might be, we need to ensure
  we link to pthreads on Linux but other platforms should work without
  extra linking.
diff: @@ -81,5 +81,14 @@ if(USE_SPGLIB)
  list(APPEND SOURCES avospglib.cpp)
  endif()
  +# The std::shared_mutex class needs pthreads on Linux.
  +if(UNIX AND NOT APPLE)
  + find_package(Threads)
  + set(EXTRA_LINK_LIB ${CMAKE_THREAD_LIBS_INIT})
  +else()
  + set(EXTRA_LINK_LIB "")
  +endif()
  +
  avogadro_add_library(AvogadroCore ${HEADERS} ${SOURCES})
  -target_link_libraries(AvogadroCore LINK_PRIVATE ${SPGLIB_LIBRARY})
  +target_link_libraries(AvogadroCore
  + LINK_PRIVATE ${SPGLIB_LIBRARY} ${EXTRA_LINK_LIB})
  