[httpd] Fix evhttp_connection_get_peer() const char warning for newer libevent

configure.ac

@@ -156,6 +156,16 @@ OWNTONE_MODULES_CHECK([OWNTONE], [LIBEVENT], [libevent >= 2],
 			[Define to 1 if you have libevent 2 (<2.1.4)])])
 	])
 
+dnl Check for evhttp_connection_get_peer() signature
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+	#include <event2/http.h>
+	void evhttp_connection_get_peer(struct evhttp_connection *, const char **, ev_uint16_t *);
+	]], [[ ]]
+	)],
+	[AC_MSG_RESULT([yes]); AC_DEFINE([HAVE_EVHTTP_CONNECTION_GET_PEER_CONST_CHAR], 1,
+		[Define to 1 if evhttp_connection_get_peer expects const char**])]
+	)
+
 OWNTONE_MODULES_CHECK([OWNTONE], [JSON_C], [json-c],
 	[json_tokener_parse], [json.h],
 	[dnl check for old version

src/httpd.c

@@ -1006,7 +1006,11 @@ httpd_request_parse(struct evhttp_request *req, struct httpd_uri_parsed *uri_par
 
       evcon = evhttp_request_get_connection(req);
       if (evcon)
+#ifdef HAVE_EVHTTP_CONNECTION_GET_PEER_CONST_CHAR
 	evhttp_connection_get_peer(evcon, &hreq->peer_address, &hreq->peer_port);
+#else
+	evhttp_connection_get_peer(evcon, (char **)&hreq->peer_address, &hreq->peer_port);
+#endif
       else
 	DPRINTF(E_LOG, L_HTTPD, "Connection to client lost or missing\n");
 

src/httpd.h

@@ -52,7 +52,7 @@ struct httpd_request {
   // http request struct (if available)
   struct evhttp_request *req;
   // Source IP address (ipv4 or ipv6) and port of the request (if available)
-  char *peer_address;
+  const char *peer_address;
   unsigned short peer_port;
   // A pointer to extra data that the module handling the request might need
   void *extra_data;