simgear/io/HTTP*: added support for enabling compression when downloading.

Uses underlying curl library's CURLOPT_ACCEPT_ENCODING.

simgear/io/HTTPClient.cxx

@@ -304,6 +304,12 @@ void Client::makeRequest(const Request_ptr& r)
         curl_easy_setopt(curlRequest, CURLOPT_RANGE, range.c_str());
         SG_LOG(SG_GENERAL, SG_DEBUG, "Have added CURLOPT_RANGE: '" << range << "'");
     }
+    
+    const char* enc = r->getAcceptEncoding();
+    if (enc) {
+        curl_easy_setopt(curlRequest, CURLOPT_ACCEPT_ENCODING, const_cast<char*>(enc));
+        SG_LOG(SG_GENERAL, SG_ALERT, "Have added CURLOPT_ACCEPT_ENCODING: '" << enc << "'");
+    }
 
     const std::string method = strutils::lowercase (r->method());
     if (method == "get") {

simgear/io/HTTPRequest.cxx

@@ -133,6 +133,24 @@ void Request::setRange(const std::string& range)
   _range = range;
 }
 
+//------------------------------------------------------------------------------
+void Request::setAcceptEncoding(const char* enc)
+{
+  if (enc) {
+    _enc_set = true;
+    _enc = enc;
+  }
+  else {
+    _enc_set = false;
+  }
+}
+
+//------------------------------------------------------------------------------
+const char* Request::getAcceptEncoding()
+{
+  return (_enc_set) ? _enc.c_str() : nullptr;
+}
+
 //------------------------------------------------------------------------------
 void Request::requestStart()
 {

simgear/io/HTTPRequest.hxx

@@ -138,6 +138,17 @@ public:
      * if we are updating a file that was fully downloaded previously.
      */
     virtual void setRange(const std::string& range);
+    
+    /*
+     * Control underlying curl library's automatic decompression support. <enc>
+     * is passed directly to curl_easy_setopt(CURLOPT_ACCEPT_ENCODING); see
+     * CURLOPT_ACCEPT_ENCODING(3) for details.
+     *
+     * E.g. pass env="" to allow any available compression algorithm.
+     */
+    virtual void setAcceptEncoding(const char* enc);
+    
+    const char* getAcceptEncoding();
 
     virtual std::string method() const
         { return _method; }
@@ -269,6 +280,8 @@ public:
     std::string   _method;
     std::string   _url;
     std::string   _range;
+    bool          _enc_set = false;
+    std::string   _enc;
     StringMap     _request_headers;
     std::string   _request_data;
     std::string   _request_media_type;