build: add configure option to generate xcode build files

Author: tjfontaine

configure

@@ -230,6 +230,11 @@ parser.add_option("--unsafe-optimizations",
     dest="unsafe_optimizations",
     help=optparse.SUPPRESS_HELP)
 
+parser.add_option("--xcode",
+    action="store_true",
+    dest="use_xcode",
+    help="Generate build files for use with xcode")
+
 (options, args) = parser.parse_args()
 
 
@@ -570,6 +575,7 @@ write('config.gypi', "# Do not edit. Generated by the configure script.\n" +
 config = {
   'BUILDTYPE': 'Debug' if options.debug else 'Release',
   'USE_NINJA': str(int(options.use_ninja or 0)),
+  'USE_XCODE': str(int(options.use_xcode or 0)),
 }
 config = '\n'.join(map('='.join, config.iteritems())) + '\n'
 
@@ -578,6 +584,8 @@ write('config.mk',
 
 if options.use_ninja:
   gyp_args = ['-f', 'ninja']
+elif options.use_xcode:
+  gyp_args = ['-f', 'xcode']
 elif os.name == 'nt':
   gyp_args = ['-f', 'msvs', '-G', 'msvs_version=auto']
 elif options.dest_os: