LLVM: Use CMake build system

This should provide exactly the same configuration as before (the CMake build
system has many options enabled by default, that previously had to be set
manually).
Add the Clang package
Clang: fix the include problem with tablegen
Julia: fix LDFLAGS handling

Now Julia compiles again.
Merge pull request #242 from hashdist/clang2

Add Clang package