add support for client linked with libpthread but not creating threads

We document that we do not support clients linking with libpthread, due to the loader and TLS interactions as well as transparency issues of creating new threads. This issue covers trying to add support for a client linking with libpthread but not actually creating new threads, which seems to come up a fair amount: https://github.com/DynamoRIO/dynamorio/issues/956#issuecomment-64847764, https://groups.google.com/forum/#!topic/dynamorio-users/BKfQKybcthU, https://groups.google.com/forum/#!topic/dynamorio-users/ssPUb8cC49Q, https://groups.google.com/forum/#!topic/dynamorio-users/ED8nbG2tIdU.