- Separate detection of lldb headers and lldb shared libraries
  since the headers can be downloaded by us while the libraries
  cannot at the moment. On some systems we are not able to
  find the non-versioned liblldb dynamic library (`-llldb`) anyway,
  it's working fine until now because the lldb will resolve the
  symbols before loading the plugin, but this will have to be handled
  differently in the case of addons which need the symbols at build time.
  Also use ldd to find the correct name of the library.
- Use the name `lldb_include_dir` and `lldb_lib_dir` and
  point them to subdirectories of the lldb installation to
  match the concepts in llvm-config.
- Check the `$lldb_include_dir/lldb/API` for headers since some
  incomplete installations may not have the API headers installed.
- Stop symlinking ./lldb because we will always run scripts/configure.js
  which can write the path to the headers in the config.gypi anyway.
- Put the conditional flags in binding.gyp to target_defaults so they
  can be shared by the addon target later.
- Add more logs in the configuration script about the result of each
  step.
- Detect llvm-config once and use it if it's available later instead of
  always trying to run it in each configuration step.