include/aces is in the include path, so don't need to specify path

This also allows using the compile tree as a cross-project cmake setup
work with the cmake Config mechanism.
fix compile with new version of OpenEXR

the new version of OpenEXR has a new namespace mechanism, so forward
declaration of things is harder, just include the header file
fix compile with modern gcc

new versions of the libraries have cleaned up includes, so not as many
headers are automatically included. Need cstdio to find sscanf
switch to using find_package for other libraries

OpenEXR and IlmBase haven't provided cmake Find*.cmake or library
Config.cmake files. This creates local versions and switches to using
them. These can probably be removed in some future release once the
upstream libraries deploy the information.
clean up duplicate code, add comment

configure.cmake already sets a global include path and library path, so
don't set it again
update the rpath settings

This should allow running locally to find the right .so files, but have
the rpath information if installing to a non-standard location. Followed
the recommended settings on the CMake FAQ for setting this
add a nuke FindNuke.cmake

This adds a FindNuke.cmake (per the TODO). The version is parsed
directly from the header, so NukeVersion.cpp is no longer needed.
Updated the documentation strings to make which setting controls which a
little more clear