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* Revert "Revert "[osx_steps] export cppflags""

This reverts commit 7956064f.

* Try using LIBRARY_PATH instead of LDFLAGS

This reverts commit 4f303070.

Prefer multibuild library locations over the defaults.
Sometimes multibuild would use an older version
over it's own.

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.