ADD: add system version compare macro.

Author: tangqiaoboy

Macros/VersionCompare.h

@@ -0,0 +1,21 @@
+/*
+ *  System Versioning Preprocessor Macros
+ */ 
+#define SYSTEM_VERSION_EQUAL_TO(v)                  ([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch] == NSOrderedSame)
+#define SYSTEM_VERSION_GREATER_THAN(v)              ([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch] == NSOrderedDescending)
+#define SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(v)  ([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch] != NSOrderedAscending)
+#define SYSTEM_VERSION_LESS_THAN(v)                 ([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch] == NSOrderedAscending)
+#define SYSTEM_VERSION_LESS_THAN_OR_EQUAL_TO(v)     ([[[UIDevice currentDevice] systemVersion] compare:v options:NSNumericSearch] != NSOrderedDescending)
+
+/*
+ *  Usage sample:
+
+if (SYSTEM_VERSION_LESS_THAN(@"4.0")) {
+    ...
+}
+
+if (SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO(@"3.1.1")) {
+    ...
+}
+
+*/