Add libiconv support

str_util.cpp

@@ -0,0 +1,15 @@
+#include "str_util.h"
+#include "cstr_util.h"
+#include <malloc.h>
+
+bool str_util::tolowercase(std::string ori, std::string& result) {
+    char* tmp = nullptr;
+    auto re = cstr_tolowercase(ori.c_str(), ori.length(), &tmp);
+    if (re) {
+        result = std::string(tmp, ori.length());
+        free(tmp);
+        return true;
+    } else {
+        return false;
+    }
+}