Fix SDK detection to work with modern versions of Xcode.

This finds the the OS X SDK directory by using xcrun and known fallbacks; users shouldn't be symlinking /Developer.

ext/toolchain/commands1.py

@@ -554,7 +554,24 @@ class InternalCommands:
  		return (major, minor, rev)
 
 	def getMacSdkDir(self):
-		return "/Developer/SDKs/MacOSX" + self.macSdk + ".sdk"
+		sdkName = "macosx" + self.macSdk
+
+		# Ideally we'll use xcrun (which is influenced by $DEVELOPER_DIR), then try a couple
+		# fallbacks to known paths if xcrun is not available
+		status, sdkPath = commands.getstatusoutput("xcrun --show-sdk-path --sdk " + sdkName)
+		if status == 0 and sdkPath:
+			return sdkPath
+
+		developerDir = os.getenv("DEVELOPER_DIR")
+		if not developerDir:
+			developerDir = "/Applications/Xcode.app/Contents/Developer"
+
+		sdkDirName = sdkName.replace("macosx", "MacOSX")
+		sdkPath = developerDir + "/Platforms/MacOSX.platform/Developer/SDKs/" + sdkDirName + ".sdk"
+		if os.path.exists(sdkPath):
+			return sdkPath
+
+		return "/Developer/SDKs/" + sdkDirName + ".sdk"
 	
 	# http://tinyurl.com/cs2rxxb
 	def fixCmakeEclipseBug(self):