Add initial project files (excluding ignored content)

external/spdlog/details/null_mutex.h

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+// Copyright(c) 2015-present, Gabi Melman & spdlog contributors.
+// Distributed under the MIT License (http://opensource.org/licenses/MIT)
+
+#pragma once
+
+#include <atomic>
+#include <utility>
+// null, no cost dummy "mutex" and dummy "atomic" int
+
+namespace spdlog {
+namespace details {
+struct null_mutex {
+    void lock() const {}
+    void unlock() const {}
+};
+
+struct null_atomic_int {
+    int value;
+    null_atomic_int() = default;
+
+    explicit null_atomic_int(int new_value)
+        : value(new_value) {}
+
+    int load(std::memory_order = std::memory_order_relaxed) const { return value; }
+
+    void store(int new_value, std::memory_order = std::memory_order_relaxed) { value = new_value; }
+
+    int exchange(int new_value, std::memory_order = std::memory_order_relaxed) {
+        std::swap(new_value, value);
+        return new_value;  // return value before the call
+    }
+};
+
+}  // namespace details
+}  // namespace spdlog