plugins: DiskIO UGens - fix cleanup of DiskIO thread

Signed-off-by: Tim Blechmann <tim@klingt.org>
common: MsgFifo - rewrite in a portable manner with c++11-style atomics

Signed-off-by: Tim Blechmann <tim@klingt.org>
sclang: TempoClock - don't declare mRun as volatile

volatile is not smp-safe. atomic<> would be the correct way. however mRun
does not need to be atomic, as it is only accessed when the language is
locked.

Signed-off-by: Tim Blechmann <tim@klingt.org>
plugins: apparently osx builds require rtti

Signed-off-by: Tim Blechmann <tim@klingt.org>