The Travis CI configurations for macOS, Clang on Linux and CheckTexinfo.py and CheckManpage.py are broken and disabled or ignored. They appear to be non-trivial to fix, so the current stub is probably more distracting than helpful while also making changes to the working Linux configuration more difficult.
In Travis CI, versions of libraries, etc. to build against are typically specified in a build matrix. In addition, currently there is no way to build against the distribution-provided Boost version. This change uses a build matrix for BOOST_VERSION and allows that variable to be empty for building against the distribution-provided Boost version.
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