Fix build failure with OS X 10.11 SDK from Xcode 7.0–7.2.1

[ryandesign]

Building textmate 2.0-rc.4 on OS X 10.10 with Xcode 7.2.1 (the last version that works on OS X 10.10) with the 10.11 SDK, the build fails:

Frameworks/OakCommand/src/OakCommand.mm:182:1: error: ARC forbids synthesizing a property of an Objective-C object with unspecified ownership or storage attribute
@implementation OakCommand
^
Frameworks/OakCommand/src/OakCommand.h:16:29: note: property declared here
@property (nonatomic) void(^modalEventLoopRunner)(OakCommand*, BOOL* didTerminate);
                            ^
Frameworks/OakCommand/src/OakCommand.mm:182:1: error: ARC forbids synthesizing a property of an Objective-C object with unspecified ownership or storage attribute
@implementation OakCommand
^
Frameworks/OakCommand/src/OakCommand.h:17:29: note: property declared here
@property (nonatomic) void(^terminationHandler)(OakCommand*, BOOL normalExit);
                            ^
2 errors generated.

Frameworks/DocumentWindow/src/SelectGrammarViewController.mm:39:1: error: ARC forbids synthesizing a property of an Objective-C object with unspecified ownership or storage attribute
@implementation SelectGrammarViewController
^
Frameworks/DocumentWindow/src/SelectGrammarViewController.mm:24:29: note: property declared here
@property (nonatomic) void(^callback)(SelectGrammarResponse, BundleGrammar*);
                            ^
1 error generated.

From my brief investigation, it seems that Xcode 7.3's 10.11 SDK (version 10.11.4) will consider these properties to be strong by default, but with Xcode 7.2.1 (10.11 SDK version 10.11.2) and earlier, it has to be specified explicitly.