macOS: clean-workspaces.sh incorrectly triggers bundled library rebuilds

[historic_bruno]

Currently in clean-workspaces.sh, it will remove bundled libraries on all *nix systems unless the --preserve-libs flag is passed:

if [ "$preserve_libs" != "true" ]; then
  echo "Cleaning bundled third-party dependencies..."

  (cd ../../libraries/source/fcollada/src && rm -rf ./output)
  (cd ../../libraries/source/nvtt/src && rm -rf ./build)
  (cd ../../libraries/source/spidermonkey && rm -f .already-built)
  (cd ../../libraries/source/spidermonkey && rm -rf ./mozjs-38.0.0)
fi

This is wrong on macOS, because the corresponding update-workspaces.sh doesn't build those libraries (as it would on Linux). Instead, the user would unexpectedly rebuild them the next time they run build-osx-libs.sh, which is very time-consuming and not desirable. The above code should not run on macOS.