Please use ./import_expat.sh to update. For example to import from a tar and rebuild:

    (croot && cd external/expat && ./import_expat.sh ~/Downloads/expat-2.1.1.tar.bz2 && mma -j32 -B)

When updating to new versions, please remove any unnecessary build
files to make the diff of what we are using clearer. It is reasonable
to keep new docs, examples, and tests for reference.