Clang: make ready for clang 3.4
Clang: updated to allow 1 stage builds

You can now choose between a 1 stage build and a full 3 stage build
(which is the default).
Clang: reworked based on review
Updated Clang easyblock to support build with polly
clang: don't use polly by default
Merge pull request #346 from wpoely86/clang34

Updated Clang block for 3.4