macx/compat: fix build of compat.m shim with Sierra SDK.

The Sierra SDK seems to require an id<NSApplicationDelegate>
for its setDelegate: call.

To accomplish this, split off the existing adoption of
NSFileManagerDelegate into a seperate class extension.

Then, add another class extension for conforming to the
NSApplicationDelegate protocol.

Both of them are gated with MAC_OS_X_VERSION_MAX_ALLOWED
to allow building with older SDKs.

macx/compat/compat.m

@@ -6,10 +6,17 @@
 #import <AppKit/AppKit.h>
 
 @interface CompatApp : NSObject
-#if MAC_OS_X_VERSION_MIN_REQUIRED >= 1070
-<NSFileManagerDelegate>
+@end
+
+#if MAC_OS_X_VERSION_MAX_ALLOWED >= 1060
+@interface CompatApp (NSApplicationDelegateExtension) <NSApplicationDelegate>
+@end
 #endif
+
+#if MAC_OS_X_VERSION_MAX_ALLOWED >= 1070
+@interface CompatApp (NSFileManagerDelegateExtension) <NSFileManagerDelegate>
 @end
+#endif
 
 @implementation CompatApp