conditional build change for clang 3.5.0

- In clang 3.5.0 libclangRewriteCore.a seems to
  no longer exist in the clang package and as such
  we conditionally include it in the build iff version
  is greater than 3.5.0.

- Hopefully should address issue #230
Merge pull request #231 from LefterisJP/master

conditional build change for clang 3.5.0