Link against libpthread.

When using system glog/protobuf/boost, the linking fails with undefined
reference for pthread_* functions.
[libprocess] Link against libpthread.

When using system glog/protobuf/boost, the linking fails with undefined
reference for pthread_* functions.
Removes #include slave/flags.hpp from isolator.hpp

Slave flags are not being used in isolator.hpp and thus slave/flags.hpp
shouldn't be included in isolator.hpp.  Instead, include it directly in only
those files that need it.
Removed redundant #include from slave/state.hpp.
Removed stale #include from type_utils.hpp.
Move Modules protobuf to mesos namespace.
Split contents of type_utils.h into {mesos,messages,log}.hpp.
Move Task and StatusUpdate from messages.proto to mesos.proto.
Compiles after exposing isolator and state.hpp.
Exposed Authentication header for building modules out of Mesos tree.