Unify love.system.getOS and love._os implementations.

src/modules/system/System.cpp

@@ -62,7 +62,7 @@ System::System()
 {
 }
 
-std::string System::getOS() const
+const char *System::getOS()
 {
 #if defined(LOVE_MACOS)
 	return "OS X";

src/modules/system/System.h

@@ -57,7 +57,7 @@ public:
 	/**
 	 * Gets the current operating system.
 	 **/
-	std::string getOS() const;
+	static const char *getOS();
 
 	/**
 	 * Gets the number of reported CPU cores on the current system.

src/modules/system/wrap_System.cpp

@@ -31,7 +31,7 @@ namespace system
 
 int w_getOS(lua_State *L)
 {
-	luax_pushstring(L, instance()->getOS());
+	luax_pushstring(L, System::getOS());
 	return 1;
 }