LibCurl未定义的引用

sudo apt-get install libcurl4-openssl-dev

#include <iostream>
#include <curl/curl.h>

int main() {
    CURL *curl;
    CURLcode res;

    curl = curl_easy_init();
    if(curl) {
        curl_easy_setopt(curl, CURLOPT_URL, "http://example.com");
        res = curl_easy_perform(curl);
        if(res != CURLE_OK)
            fprintf(stderr, "curl_easy_perform() failed: %s\n", curl_easy_strerror(res));
        curl_easy_cleanup(curl);
    }
    return 0;
}

g++ example.cpp -o example -lcurl

g++ example.cpp -o example -L/usr/local/lib -lcurl

g++ example.cpp -o example -I/usr/local/include -L/usr/local/lib -lcurl

cmake_minimum_required(VERSION 3.0)
project(Example)

find_package(CURL REQUIRED)
include_directories(${CURL_INCLUDE_DIR})

add_executable(example example.cpp)
target_link_libraries(example ${CURL_LIBRARIES})

mkdir build
cd build
cmake ..
make