bb#11106 - with-system-llvm clarity:
added additional information to llvm summary
added defaults "/usr/local" and "/usr" to with-system-llvm
enable-llvm defaults to "yes" if with-system-llvm specified, "auto" otherwise
updated help messages, added with-system-llvm to main configure help
autojunk'd
bc: bb#11107 - clarify meesages where LLVM is not compiled or not linked