build: Add msgpack as git submodule

third_party/CMakeLists.txt

@@ -3,3 +3,4 @@
 
 add_subdirectory(simplecrypt)
 add_subdirectory(qjsonwebtoken)
+add_subdirectory(cmake/msgpack EXCLUDE_FROM_ALL SYSTEM)

third_party/cmake/msgpack/CMakeLists.txt

@@ -0,0 +1,12 @@
+# SPDX-FileCopyrightText: Copyright (C) swift Project Community / Contributors
+# SPDX-License-Identifier: GPL-3.0-or-later OR LicenseRef-swift-pilot-client-1
+
+# for simplicity, we are using our own CMake file to provide the msgpack-cxx target
+
+add_library(msgpack-cxx INTERFACE)
+
+if(NOT EXISTS ${PROJECT_SOURCE_DIR}/third_party/msgpack/include/)
+    message(FATAL_ERROR "msgpack not found. Did you forget to initialize the git submodules?")
+endif()
+
+target_include_directories(msgpack-cxx INTERFACE ${PROJECT_SOURCE_DIR}/third_party/msgpack/include)