build: Don't change CMAKE_XXX_OUTPUT_DIRECTORY when built as subdir

[touraill-adobe]

When building OIIO as a subdirectory of a super-project (with add_subdirectory), it is better to honor the output directories specified by the parent project.

In our project, it was causing issues because OIIO libs were ending in a different folder from USD plugins, which thereby failed to load correctly in the build tree.

• [x] I have read the contribution guidelines.
• [ ] I have updated the documentation, if applicable.
• [ ] I have ensured that the change is tested somewhere in the testsuite (adding new test cases if necessary).
• [ ] If I added or modified a C++ API call, I have also amended the corresponding Python bindings (and if altering ImageBufAlgo functions, also exposed the new functionality as oiiotool options).
• [x] My code follows the prevailing code style of this project. If I haven't already run clang-format before submitting, I definitely will look at the CI test that runs clang-format and fix anything that it highlights as being nonconforming.

[linux-foundation-easycla[bot]]

The committers listed above are authorized under a signed CLA.

• :white_check_mark: login: touraill-adobe / name: Luc Touraille (ec68dac3a168d2e4a23da707bd930e0c968422fc)

[lgritz]

Hi, before we merge, we need a CLA on file from you, and also the commit needs to be "signed" to indicate acceptance of the required DCO. You can do the latter simply by

git commit --amend -s
git push origin <branchname> --force

[touraill-adobe]

Hi, before we merge, we need a CLA on file from you, and also the commit needs to be "signed" to indicate acceptance of the required DCO. You can do the latter simply by

git commit --amend -s
git push origin <branchname> --force

Yes I'm waiting for the CLA to be signed by my company :)

Sorry about the missing sign-off, I missed the requirement when reading the guidelines, I updated the commit.

[lgritz]

Sorry, I didn't notice that the CLA had been fixed up here. Merging.