build: Don't change CMAKE_XXX_OUTPUT_DIRECTORY when built as subdir

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.

CMakeLists.txt

@@ -87,9 +87,11 @@ message (STATUS "Build type             = ${CMAKE_BUILD_TYPE}")
 message (STATUS "Supported release      = ${${PROJECT_NAME}_SUPPORTED_RELEASE}")
 
 # Make the build area layout look a bit more like the final dist layout
-set (CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
-set (CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
-set (CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
+if (PROJECT_IS_TOP_LEVEL)
+    set (CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
+    set (CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
+    set (CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
+endif()
 
 if ("${PROJECT_SOURCE_DIR}" STREQUAL "${CMAKE_BINARY_DIR}")
     message (FATAL_ERROR "Not allowed to run in-source build!")