Reinstate CPR using system library.

vendor/CPR/include/cpr/curlholder.h

@@ -0,0 +1,50 @@
+#ifndef CPR_CURL_HOLDER_H
+#define CPR_CURL_HOLDER_H
+
+#include <array>
+#include <mutex>
+#include <string>
+
+#include <curl/curl.h>
+
+namespace cpr {
+struct CurlHolder {
+  private:
+    /**
+     * Mutex for curl_easy_init().
+     * curl_easy_init() is not thread save.
+     * References:
+     * https://curl.haxx.se/libcurl/c/curl_easy_init.html
+     * https://curl.haxx.se/libcurl/c/threadsafe.html
+     **/
+    // It does not make sense to make a std::mutex const.
+    // NOLINTNEXTLINE (cppcoreguidelines-avoid-non-const-global-variables)    
+    static std::mutex curl_easy_init_mutex_;
+
+  public:
+    CURL* handle{nullptr};
+    struct curl_slist* chunk{nullptr};
+    struct curl_httppost* formpost{nullptr};
+    std::array<char, CURL_ERROR_SIZE> error{};
+
+    CurlHolder();
+    CurlHolder(const CurlHolder& other) = default;
+    CurlHolder(CurlHolder&& old) noexcept = default;
+    ~CurlHolder();
+
+    CurlHolder& operator=(CurlHolder&& old) noexcept = default;
+    CurlHolder& operator=(const CurlHolder& other) = default;
+
+    /**
+     * Uses curl_easy_escape(...) for escaping the given string.
+     **/
+    std::string urlEncode(const std::string& s) const;
+
+    /**
+     * Uses curl_easy_unescape(...) for unescaping the given string.
+     **/
+    std::string urlDecode(const std::string& s) const;
+};
+} // namespace cpr
+
+#endif