Shut up some warnings

src/libmain/stack.cc

@@ -32,7 +32,7 @@ static void sigsegvHandler(int signo, siginfo_t * info, void * ctx)
         if (diff < 0) diff = -diff;
         if (diff < 4096) {
             char msg[] = "error: stack overflow (possible infinite recursion)\n";
-            (void) write(2, msg, strlen(msg));
+            [[gnu::unused]] int res = write(2, msg, strlen(msg));
             _exit(1); // maybe abort instead?
         }
     }

src/libstore/local-store.cc

@@ -124,7 +124,7 @@ LocalStore::LocalStore(const Params & params)
 #endif
             if (res == -1) {
                 writeFull(fd.get(), string(settings.reservedSize, 'X'));
-                ftruncate(fd.get(), settings.reservedSize);
+                [[gnu::unused]] auto res2 = ftruncate(fd.get(), settings.reservedSize);
             }
         }
     } catch (SysError & e) { /* don't care about errors */