Change detection logic for Objects-normal directory

Per https://github.com/imobile3/cassowary/pull/2:

"Right now build_dir_output looks for a hard-coded directory named 'UnitTests.build'. Instead, use xcodebuild with some arguments to figure out where the files really ended up. I'm not sure if OBJECT_FILE_DIR_normal is a newer variable nor not, so I also had it fall back to TARGET_TEMP_DIR'"