Generator for LLVM::CONFIG module, cleaned up

This pull request is a follow up for #14.

 * I've fixed a typo preventing the extension/config module from being built.
 * I've removed all config options related to LLVM source or libraries. Ruby-LLVM requires LLVM to be built as a monolithic .so, and it is definitely redistributed without LLVM sources, hence there is no use case for these options.
 * I've also removed LLVM::CONFIG::VERSION, as there's already LLVM::LLVM_VERSION.