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--enable-unsupported is necesssary for HDF5 to build with both
threadsafety and high level library enabled.

--with-pthreads form HDF5 docs and conda-forge

Thread safety flags copied from conda-forge hdf5-feedstock:
conda-forge/hdf5-feedstock@36304eabd09a68d60a726fd6de4913d38c040917

Build HDF5 with --enable-thread-safe flag.

Since https://github.com/travis-ci/travis-build/commit/52c6b6cc1d537e52c8ae6f0906944d6fcaa3d173, it's no longer needed

It went down, I noticed yesterday.

We can't easily predict what the output directory will be for a Github
release archive.  Get fetch_unpack to tell us.  Add some tests that
won't fail, but at least show us the output.

https://travis-ci.org/matthew-brett/multibuild/jobs/382564348#L2248

Resolution option 2-1 for https://github.com/matthew-brett/multibuild/issues/116

Add libtool, flex, and bison.

flex must be fetched from the released archive, rather
than the GitHub autogenerated source.

The archives directory was not created. I tested this downstream and the fix worked.