This adds a basic unit test to check USD has been correctly
build with imaging components to support building both with
the old and new libs, it automatically adds the test when it
detects a library with imaging enabled. (platform devs will
have to pay attention it runs the test to validate the libs
build correctly)
For future use in the code it also defines a USD_HAS_IMAGING
define one could check if we're building against an USD lib
that has it (just because we build/ship with it, doesn't
mean downstream builds will ship with it, so we'll have
to be a little pro-active there)
Reviewed By: sybren
Differential Revision:https://developer.blender.org/D14456
b9f1b64801