[FEATURE REQUEST] CMake Subdirectory Support

**Is your feature request related to a problem? Please describe.**
In my projects, I prefer to have all the dependencies as submodules, which are then included in the projects CMake build system by ``add_subdirectory``. However, it seems that this does not work with oiio.

I think that this is due to lines such as [128](https://github.com/OpenImageIO/oiio/blob/6307f02457311427515b83e0912e8fefd8a6f53c/CMakeLists.txt#L128), where it requires oiio to be built in ``CMAKE_BINARY_DIR``, which may not be the case if it is in a subdirectory.

**Describe the solution you'd like**
Reconfiguration of the CMake files such that they do not rely directly on ``CMAKE_BINARY_DIR``, but instead use ``CMAKE_CURRENT_BINARY_DIR``, so that the project can be compiled as a dependency of other projects.

Start at CMakeLists.txt line 128 and inspect the CMake files for uses of CMAKE_BINARY_DIR. Configure OpenImageIO as an add_subdirectory dependency and verify that it compiles without relying on the parent project's binary directory.

Written by the indexing model from the issue text.