These SDL_* return NULL on error, and gcc11/clang won't allow this.
https://bugs.gentoo.org/739086
--- a/client_net.cpp
+++ b/client_net.cpp
@@ -33,3 +33,3 @@
   cond = SDL_CreateCond();
-  if (cond <0)
+  if (!cond)
     {
@@ -39,3 +39,3 @@
   mutex = SDL_CreateMutex();
-  if (mutex < 0)
+  if (!mutex)
     {
@@ -47,3 +47,3 @@
   trans_th = SDL_CreateThread (&client_tcpnet::transf_func, (void *) this);
-  if (trans_th < 0)
+  if (!trans_th)
     {
@@ -53,3 +53,3 @@
   input_th = SDL_CreateThread (&client_tcpnet::input_func, (void *) this);
-  if (input_th < 0)
+  if (!input_th)
     {
--- a/game_server.cpp
+++ b/game_server.cpp
@@ -66,3 +66,3 @@
   con_th = SDL_CreateThread (&game_server::connection_accepter, (void *) this);
-  if (con_th < 0)
+  if (!con_th)
     {