Make IDE build on OSX when 10.6 SDK isn't available

- More recent versions of Xcode no longer ship with the
  10.6 SDK so the default SDK setting of 10.6 causes an
  error. This change makes it build with whatever is
  the latest SDK on whatever Xcode is being used
  but sets the deployment target to 10.6 so that it
  will run there as well.

IDE/Build/Mac OS X/Polycode.xcodeproj/project.pbxproj

@@ -518,6 +518,7 @@
 					"\"$(SRCROOT)/../../../Release/Darwin/Framework/Core/Dependencies/lib\"",
 					"\"$(SRCROOT)/../../../Release/Darwin/Framework/Modules/lib\"",
 				);
+				MACOSX_DEPLOYMENT_TARGET = 10.6;
 				PRODUCT_NAME = Polycode;
 			};
 			name = Debug;
@@ -556,6 +557,7 @@
 					"\"$(SRCROOT)/../../../Release/Darwin/Framework/Core/Dependencies/lib\"",
 					"\"$(SRCROOT)/../../../Release/Darwin/Framework/Modules/lib\"",
 				);
+				MACOSX_DEPLOYMENT_TARGET = 10.6;
 				PRODUCT_NAME = Polycode;
 			};
 			name = Release;
@@ -581,7 +583,7 @@
 				);
 				ONLY_ACTIVE_ARCH = YES;
 				PREBINDING = NO;
-				SDKROOT = macosx10.6;
+				SDKROOT = macosx;
 			};
 			name = Debug;
 		};
@@ -605,7 +607,7 @@
 				);
 				ONLY_ACTIVE_ARCH = YES;
 				PREBINDING = NO;
-				SDKROOT = macosx10.6;
+				SDKROOT = macosx;
 			};
 			name = Release;
 		};