build on osx with clang

[cottrell]

In case it is useful to anyone, managed to build and run the example on recent osx (clang) with a couple of minor patches. See https://github.com/cottrell/difacto/tree/osx

709eb4b
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$ gcc --version
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 7.3.0 (clang-703.0.29)
Target: x86_64-apple-darwin15.3.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin