Review: Added OCIO_VERSION_HEX to public header for easy version comparison

Jeremy Selan <jeremy...@...>

This exposes a single 4-byte hex number, e.g. 0x01050200 == 1.5.2

Use this for numeric comparisons, e.g. #if OCIO_VERSION_HEX >= ...
Note: in the case where SOVERSION is overridden at compile-time, this
will reflect the original API version number.

Code concept is stolen from cpython source.

Addresses issue

-- Jeremy