build: automatically generate list of library_files

Automatically generate the list of library_files by using Python's os.walk so we don't have to remember to specify new files (or remember to remove old ones.)

Example output:

# Do not edit. Generated by the configure script.
{ 'target_defaults': { 'cflags': [],
                       'default_configuration': 'Release',
                       'defines': [],
                       'include_dirs': [],
                       'libraries': []},
  'variables': { 'asan': 0,
                 'host_arch': 'x64',
                 'icu_small': 'false',
                 'library_files': [ 'src/node.js',
                                    'lib/_debug_agent.js',
                                    'lib/_debugger.js',
                                    'lib/_http_agent.js',
                                    'lib/_http_client.js',
                                    'lib/_http_common.js',
                                    'lib/_http_incoming.js',
                                    'lib/_http_outgoing.js',
                                    'lib/_http_server.js',
                                    'lib/_linklist.js',
                                    'lib/_stream_duplex.js',
                                    'lib/_stream_passthrough.js',
                                    'lib/_stream_readable.js',
                                    'lib/_stream_transform.js',
                                    'lib/_stream_wrap.js',
                                    'lib/_stream_writable.js',
                                    'lib/_tls_common.js',
                                    'lib/_tls_legacy.js',
                                    'lib/_tls_wrap.js',
                                    'lib/assert.js',
                                    'lib/buffer.js',
                                    'lib/child_process.js',
                                    'lib/cluster.js',
                                    'lib/console.js',
                                    'lib/constants.js',
                                    'lib/crypto.js',
                                    'lib/dgram.js',
                                    'lib/dns.js',
                                    'lib/domain.js',
                                    'lib/events.js',
                                    'lib/freelist.js',
                                    'lib/fs.js',
                                    'lib/http.js',
                                    'lib/https.js',
                                    'lib/module.js',
                                    'lib/net.js',
                                    'lib/os.js',
                                    'lib/path.js',
                                    'lib/process.js',
                                    'lib/punycode.js',
                                    'lib/querystring.js',
                                    'lib/readline.js',
                                    'lib/repl.js',
                                    'lib/stream.js',
                                    'lib/string_decoder.js',
                                    'lib/sys.js',
                                    'lib/timers.js',
                                    'lib/tls.js',
                                    'lib/tty.js',
                                    'lib/url.js',
                                    'lib/util.js',
                                    'lib/v8.js',
                                    'lib/vm.js',
                                    'lib/zlib.js',
                                    'lib/internal/child_process.js',
                                    'lib/internal/freelist.js',
                                    'lib/internal/linkedlist.js',
                                    'lib/internal/module.js',
                                    'lib/internal/readme.md',
                                    'lib/internal/repl.js',
                                    'lib/internal/socket_list.js',
                                    'lib/internal/util.js',
                                    'lib/internal/streams/lazy_transform.js'],
                 'llvm_version': 0,
                 'node_byteorder': 'little',
                 'node_install_npm': 'true',
                 'node_prefix': '/usr/local',
                 'node_release_urlbase': '',
                 'node_shared_http_parser': 'false',
                 'node_shared_libuv': 'false',
                 'node_shared_openssl': 'false',
                 'node_shared_zlib': 'false',
                 'node_tag': '',
                 'node_use_dtrace': 'true',
                 'node_use_etw': 'false',
                 'node_use_lttng': 'false',
                 'node_use_openssl': 'true',
                 'node_use_perfctr': 'false',
                 'openssl_fips': '',
                 'openssl_no_asm': 0,
                 'python': '/usr/local/opt/python/bin/python2.7',
                 'target_arch': 'x64',
                 'uv_parent_path': '/deps/uv/',
                 'uv_use_dtrace': 'true',
                 'v8_enable_gdbjit': 0,
                 'v8_enable_i18n_support': 0,
                 'v8_no_strict_aliasing': 1,
                 'v8_optimized_debug': 0,
                 'v8_random_seed': 0,
                 'v8_use_snapshot': 1,
                 'want_separate_host_toolset': 0}}