Add async library to the project

.gitignore

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+build/*
+build-*
+.vscode/*.db
+.vscode/settings.json
+third_party
+genfiles/*
+*.sublime-*
+.tags
+!third_party/include/*
+*.pyc
+/CMakeLists.txt.user
+_deps

.gitmodules

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+[submodule "async"]
+	path = async
+	url = https://github.com/romange/async.git

CMakeLists.txt

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+cmake_minimum_required(VERSION 3.15 FATAL_ERROR)
+set(PROJECT_CONTACT romange@gmail.com)
+
+enable_testing()
+
+set(CMAKE_EXPORT_COMPILE_COMMANDS 1)
+
+# Set targets in folders
+set_property(GLOBAL PROPERTY USE_FOLDERS ON)
+project(DRAGONFLY C CXX)
+set(CMAKE_CXX_STANDARD 17)
+
+# We must define all the required variables from the root cmakefile, otherwise
+# they just disappear.
+set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/async/cmake" ${CMAKE_MODULE_PATH})
+option(BUILD_SHARED_LIBS "Build shared libraries" OFF)
+
+include(third_party)
+include(internal)
+
+Message(STATUS "THIRD_PARTY_LIB_DIR ${THIRD_PARTY_LIB_DIR}")
+
+include_directories(${CMAKE_CURRENT_SOURCE_DIR})
+include_directories(async)
+
+add_subdirectory(async)

async

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+Subproject commit 39be1827d923881c662a684a215cb2dbba08d230