make the extupdate build conditional

extupdate is not very useful for most platforms, so only
build it if --enable-extupdate is passed to configure


Origin commit data
------------------
Commit: 5b72a7fe3c
Author: Stefan Seyfried <seife@tuxbox-git.slipkontur.de>
Date: 2013-02-22 (Fri, 22 Feb 2013)

src/system/helpers.cpp

@@ -409,8 +409,10 @@ bool CFileHelpers::copyDir(const char *Src, const char *Dst, bool backupMode)
 			// is file
 			else if (S_ISREG(FileInfo.st_mode)) {
 				std::string save = "";
+#if ENABLE_EXTUPDATE
 				if (backupMode && (CExtUpdate::getInstance()->isBlacklistEntry(srcPath)))
 					save = ".save";
+#endif
 				copyFile(srcPath, (dstPath + save).c_str(), FileInfo.st_mode & 0x0FFF);
 			}
 		}