honor the $XDG_CACHE_HOME env variable

Descent3/init.cpp

@@ -1985,13 +1985,7 @@ void SetupTempDirectory(void) {
   if (t_arg) {
     Descent3_temp_directory = GameArgs[t_arg + 1];
   } else {
-    std::error_code ec;
-    std::filesystem::path tempPath = std::filesystem::temp_directory_path(ec);
-    if (ec) {
-      Error("Could not find temporary directory: \"%s\"", ec.message().c_str());
-      exit(1);
-    }
-    Descent3_temp_directory = tempPath / "Descent3" / "cache";
+    Descent3_temp_directory = ddio_GetTempPath();
   }
 
   std::error_code ec;

ddio/ddio.h

@@ -426,4 +426,10 @@ bool ddio_CreateLockFile(const std::filesystem::path &dir);
  */
 bool ddio_DeleteLockFile(const std::filesystem::path &dir);
 
+/**
+ * Gets path where to write temporary/cache files.
+ * @return path where user can write cache files.
+ */
+std::filesystem::path ddio_GetTempPath();
+
 #endif

ddio/file.cpp

@@ -214,3 +214,30 @@ std::filesystem::path ddio_GetBasePath() {
   std::filesystem::path result = std::filesystem::canonical(exe_path);
   return result;
 }
+
+std::filesystem::path ddio_GetTempPath() {
+  std::filesystem::path result;
+
+#if defined(POSIX)
+  const char *envr = SDL_getenv("XDG_CACHE_HOME");
+  if (envr) {
+    result = std::filesystem::path(envr) / "Descent3";
+  } else {
+    envr = SDL_getenv("HOME");
+    if (envr) {
+      result = std::filesystem::path(envr) / ".cache" / "Descent3";
+    } else {
+#endif
+      std::error_code ec;
+      std::filesystem::path tempPath = std::filesystem::temp_directory_path(ec);
+      if (ec) {
+        Error("Could not find temporary directory: \"%s\"", ec.message().c_str());
+        exit(1);
+      }
+      result = tempPath / "Descent3" / "cache";
+#if defined(POSIX)
+    }
+  }
+#endif
+  return result;
+}