Most of the platform defintions in include/boost/thread/detail/platform.hpp are unused

Perhaps it would be a good idea to remove the unused definitions and switch to using Boost.Predef for detection where needed instead?

Example:
```
jking@ubuntu:~/boost/libs/thread$ grep -r AMIGAOS .
./include/boost/thread/detail/platform.hpp:# define BOOST_THREAD_AMIGAOS
jking@ubuntu:~/boost/libs/thread$ grep -r THREAD_SOL .
./include/boost/thread/detail/platform.hpp:# define BOOST_THREAD_SOLARIS
jking@ubuntu:~/boost/libs/thread$ grep -r THREAD_LINUX .
./include/boost/thread/detail/platform.hpp:# define BOOST_THREAD_LINUX
jking@ubuntu:~/boost/libs/thread$ grep -r THREAD_BSD .
./include/boost/thread/detail/platform.hpp:# define BOOST_THREAD_BSD
```