DEV-34837: Allow Mac SL executable to find libllcommon.dylib in app bundle's

Resources subdir without requiring user to explicitly set DYLD_LIBRARY_PATH.
Engage the SHARED_LIB_STAGING_DIR mechanism for Mac as well as Windows and
Linux so unit-test executables, too, can find libllcommon.dylib. Perform the
POST_BUILD copy of libllcommon.dylib to SHARED_LIB_STAGING_DIR everywhere but
Windows -- but make it conditional on SHARED_LIB_STAGING_DIR.