Remove clang_includes folder
Don't pass extra include path to libclang
Add libclang builtin headers for LLVM release 3.3 and 3.4

Copied from the ./lib/clang/3.X/include directories of the prebuilt binary releases at llvm.org, specifically
http://llvm.org/releases/3.3/clang+llvm-3.3-amd64-Ubuntu-12.04.2.tar.gz and http://llvm.org/releases/3.4/clang+llvm-3.4-x86_64-linux-gnu-ubuntu-13.10.tar.xz.

These headers don't appear to be arch dependent, so we should be good.
Add builtin includes for AppleClang 5.1

Copied from Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/lib
Merge pull request #910 from nickhutchinson/fix-libclang-includes

Fix libclang's #include search path