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library_builders.sh script in fact needs common_utils.sh.  Source that
script, and put a guard in common_utils.sh to prevent it being sourced
more than once.

For getting versions from versions with suffixes.

Wrong indentation causing error in variable definition - see
https://github.com/matthew-brett/manylinux-builds/pull/10#issuecomment-233305951

[skip ci]

BUILD_COMMIT not available directly from the repository, and may be useful for
later build steps.

[skip ci]

[skip ci]

From suggestion by Ralf G.

[skip ci]

Always load library builders (for OSX as well as Manylinux).

The thing that sets the Python version for building and testing is the
MB_PYTHON_VERSION variable.

Build / test Python versions conceptual separable in case of Linux
builds.

Thanks to Uwe L. Korn for reminding me.

Useful for scripts overriding "build_wheel" function.

Make function to download and unpack archives.

This allows caching the archives in the `archives` directory, either by
committing to the git repo, or when doing repeated builds of the same
library.

Also, enable encoding on basis of
https://gitlab.dkrz.de/k202009/libaec/blob/master/doc/license.txt

Get the main platform architecture from uname.

Slight generalization of wheel building for matplotlib case where
versioneer gets confused.

Make configuration more general.

Move responsibility for source futzing back to the travis.yml file,
because we are now doing only one build per job.