Actually pass lambda as a function pointer

It seems like lambdas DON'T automatically decay to a function pointer
with C varargs functions. Add an explicit + to do so.

lib/request.cpp

@@ -217,20 +217,23 @@ Request &Request::connection_timeout(long t) {
 
     this->connection_timeout_ = t;
 #ifdef TCP_USER_TIMEOUT
-    curl_easy_setopt(this->easy, CURLOPT_SOCKOPTFUNCTION, [](void *clientp, curl_socket_t curlfd, curlsocktype) -> int {
-        unsigned int val = static_cast<Request *>(clientp)->connection_timeout_ * 1000 /*ms*/;
-        setsockopt(curlfd, SOL_TCP, TCP_USER_TIMEOUT, (const char *)&val, sizeof(val));
-        return CURLE_OK;
-    });
-    curl_easy_setopt(this->easy, CURLOPT_SOCKOPTDATA, this);
+    // The + is needed to convert this to a function pointer!
+    curl_easy_setopt(
+        this->easy, CURLOPT_SOCKOPTFUNCTION, +[](void *clientp, curl_socket_t curlfd, curlsocktype) -> int {
+            unsigned int val = static_cast<Request *>(clientp)->connection_timeout_ * 1000 /*ms*/;
+            setsockopt(curlfd, SOL_TCP, TCP_USER_TIMEOUT, (const char *)&val, sizeof(val));
+            return CURLE_OK;
+        });
 #elif defined(TCP_MAXRT)
-    curl_easy_setopt(this->easy, CURLOPT_SOCKOPTFUNCTION, [](void *clientp, curl_socket_t sock, curlsocktype) -> int {
-        unsigned int maxrt = static_cast<Request *>(clientp)->connection_timeout_ /*s*/;
-        setsockopt(sock, IPPROTO_TCP, TCP_MAXRT, (const char *)&maxrt, sizeof(maxrt));
-        return CURLE_OK;
-    });
-    curl_easy_setopt(this->easy, CURLOPT_SOCKOPTDATA, this);
+    // The + is needed to convert this to a function pointer!
+    curl_easy_setopt(
+        this->easy, CURLOPT_SOCKOPTFUNCTION, +[](void *clientp, curl_socket_t sock, curlsocktype) -> int {
+            unsigned int maxrt = static_cast<Request *>(clientp)->connection_timeout_ /*s*/;
+            setsockopt(sock, IPPROTO_TCP, TCP_MAXRT, (const char *)&maxrt, sizeof(maxrt));
+            return CURLE_OK;
+        });
 #endif
+    curl_easy_setopt(this->easy, CURLOPT_SOCKOPTDATA, this);
 
     return *this;
 }