Add Boost.Atomic dependencies

.gitmodules

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+[submodule "third_party/boost_1_86_0/libs/config"]
+	path = third_party/boost_1_86_0/libs/config
+	url = https://github.com/boostorg/config
+[submodule "third_party/boost_1_86_0/libs/predef"]
+	path = third_party/boost_1_86_0/libs/predef
+	url = https://github.com/boostorg/predef
+[submodule "third_party/boost_1_86_0/libs/preprocessor"]
+	path = third_party/boost_1_86_0/libs/preprocessor
+	url = https://github.com/boostorg/preprocessor
+[submodule "third_party/boost_1_86_0/libs/winapi"]
+	path = third_party/boost_1_86_0/libs/winapi
+	url = https://github.com/boostorg/winapi
+[submodule "third_party/boost_1_86_0/libs/assert"]
+	path = third_party/boost_1_86_0/libs/assert
+	url = https://github.com/boostorg/assert
+[submodule "third_party/boost_1_86_0/libs/core"]
+	path = third_party/boost_1_86_0/libs/core
+	url = https://github.com/boostorg/core
+[submodule "third_party/boost_1_86_0/libs/static_assert"]
+	path = third_party/boost_1_86_0/libs/static_assert
+	url = https://github.com/boostorg/static_assert
+[submodule "third_party/boost_1_86_0/libs/throw_exception"]
+	path = third_party/boost_1_86_0/libs/throw_exception
+	url = https://github.com/boostorg/throw_exception
+[submodule "third_party/boost_1_86_0/libs/align"]
+	path = third_party/boost_1_86_0/libs/align
+	url = https://github.com/boostorg/align
+[submodule "third_party/boost_1_86_0/libs/type_traits"]
+	path = third_party/boost_1_86_0/libs/type_traits
+	url = https://github.com/boostorg/type_traits
+[submodule "third_party/boost_1_86_0/libs/atomic"]
+	path = third_party/boost_1_86_0/libs/atomic
+	url = https://github.com/boostorg/atomic

cmake/FindBoost.Atomic.cmake

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+if (PREVIEW_USE_SYSTEM_BOOST)
+    if ("${PREVIEW_BOOST_PATH}" STREQUAL "")
+        find_package(Boost REQUIRED COMPONENTS atomic)
+    else ()
+        find_package(Boost REQUIRED COMPONENTS atomic PATHS "${PREVIEW_BOOST_PATH}")
+    endif ()
+else()
+    add_subdirectory("${PREVIEW_THIRD_PARTY_DIR}/boost_1_86_0/libs/config"
+                     "${CMAKE_CURRENT_BINARY_DIR}/_deps/boost/config")
+    add_subdirectory("${PREVIEW_THIRD_PARTY_DIR}/boost_1_86_0/libs/predef"
+                     "${CMAKE_CURRENT_BINARY_DIR}/_deps/boost/predef")
+    add_subdirectory("${PREVIEW_THIRD_PARTY_DIR}/boost_1_86_0/libs/preprocessor"
+                     "${CMAKE_CURRENT_BINARY_DIR}/_deps/boost/preprocessor")
+    add_subdirectory("${PREVIEW_THIRD_PARTY_DIR}/boost_1_86_0/libs/assert"
+                     "${CMAKE_CURRENT_BINARY_DIR}/_deps/boost/assert")
+    add_subdirectory("${PREVIEW_THIRD_PARTY_DIR}/boost_1_86_0/libs/static_assert"
+                     "${CMAKE_CURRENT_BINARY_DIR}/_deps/boost/static_assert")
+    add_subdirectory("${PREVIEW_THIRD_PARTY_DIR}/boost_1_86_0/libs/throw_exception"
+                     "${CMAKE_CURRENT_BINARY_DIR}/_deps/boost/throw_exception")
+    add_subdirectory("${PREVIEW_THIRD_PARTY_DIR}/boost_1_86_0/libs/core"
+                     "${CMAKE_CURRENT_BINARY_DIR}/_deps/boost/core")
+    add_subdirectory("${PREVIEW_THIRD_PARTY_DIR}/boost_1_86_0/libs/align"
+                     "${CMAKE_CURRENT_BINARY_DIR}/_deps/boost/align")
+    add_subdirectory("${PREVIEW_THIRD_PARTY_DIR}/boost_1_86_0/libs/type_traits"
+                     "${CMAKE_CURRENT_BINARY_DIR}/_deps/boost/type_traits")
+    if (WIN32)
+        add_subdirectory("${PREVIEW_THIRD_PARTY_DIR}/boost_1_86_0/libs/winapi"
+                         "${CMAKE_CURRENT_BINARY_DIR}/_deps/boost/winapi")
+    endif ()
+    add_subdirectory("${PREVIEW_THIRD_PARTY_DIR}/boost_1_86_0/libs/atomic"
+                     "${CMAKE_CURRENT_BINARY_DIR}/_deps/boost/atomic")
+endif ()

third_party/BOOST.md

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+# Boost dependencies
+
+Since C++20 `preview::atomic` uses `std::atomic` by default, but
+`boost::atomic` is used before C++20. There are some several options to adjust Boost settings:
+
+***`PREVIEW_USE_BOOST_ATOMIC`***
+* Always use `boost::atomic` instead of `std::atomic`.
+* Default: `OFF` if compiler provides C++20 `std::atomic`, `ON` otherwise.
+  * Note: This doesn't check C++ version(i.e., latest AppleClang provides C++20 atomic before C++20)
+
+***`PREVIEW_USE_SYSTEM_BOOST`***
+* Use system Boost library instead of bundled Boost.
+* Default: `OFF`
+
+***`PREVIEW_BOOST_PATH`***
+* Path to Boost library. It will try to find Boost library from system paths if not set.
+* Default: Empty string

third_party/boost_1_86_0/LICENSE_1_0.txt

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+Boost Software License - Version 1.0 - August 17th, 2003
+
+Permission is hereby granted, free of charge, to any person or organization
+obtaining a copy of the software and accompanying documentation covered by
+this license (the "Software") to use, reproduce, display, distribute,
+execute, and transmit the Software, and to prepare derivative works of the
+Software, and to permit third-parties to whom the Software is furnished to
+do so, all subject to the following:
+
+The copyright notices in the Software and this entire statement, including
+the above license grant, this restriction and the following disclaimer,
+must be included in all copies of the Software, in whole or in part, and
+all derivative works of the Software, unless such copies or derivative
+works are solely in the form of machine-executable object code generated by
+a source language processor.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
+SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
+FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
+ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.