clang-pure: Pure C++ code analysis with libclang

Warnings:

• 'ghc-options: -O2' is rarely needed. Check that it is giving a real benefit and not just imposing longer compile times on your users.
• From version 1.23 cabal supports specifiying explicit dependencies for Custom setup scripts. Consider using cabal-version >= 1.23 and adding a 'custom-setup' section with a 'setup-depends' field that specifies the dependencies of the Setup.hs script itself. The 'setup-depends' field uses the same syntax as 'build-depends', so a simple example would be 'setup-depends: base, Cabal'.