Refactor the OIIO_DISPATCH macros.
Previously, they cause a return. Now they set a variable with the return
value. This is helpful for functions that want to dispatch, and then
do more work without a forced return.
Changed -Wunused-private-field to -Wno-unused-private-field, now builds with Qt (iv)
Fixes for Boost Filesystem in 1.46-1.49.
Compile for C++11 with make USE_CPP11=1 and with libc++ as USE_LIBCPLUSPLUS=1