Bindings: Try to find libclang4 before trying to find libclang3

The LLVM project has picked a new version scheme

http://blog.llvm.org/2016/12/llvms-new-versioning-scheme.html

which means that major version numbers will be bumped more regularly,
the minor version will remain at zero, and the patch version will be
bumped. We will have to see how distros respond in terms of file
naming.