Make IB iterators return valid black pixel data for missing tiles.
It's a lot better than crashing when you dereference a NULL pointer!
SPI-specific internal build instruction changes from Sosh
SPI customization: Allow specification of PYVER from the command line
SPI customizations: fix incorrect linkage of python libs to lib/libOpenImageIO.so
We only want to link against python for the python module python/OpenImageIO.so
Clean up some other Python handling in SPI's custom build setup, including being
able ot specify a different python verion with the PYVER environment variable.