GHA: Add CMake subdir test

test/cmake_test/CMakeLists.txt

@@ -0,0 +1,22 @@
+# Copyright 2021-2024 Alexander Grund
+# Distributed under the Boost Software License, Version 1.0.
+# See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt
+
+cmake_minimum_required(VERSION 3.5...3.16)
+
+project(cmake_subdir_test LANGUAGES CXX)
+
+# Those 2 should work the same
+# while using find_package for the installed Boost avoids the need to manually specify dependencies
+if(BOOST_CI_INSTALL_TEST)
+    find_package(boost_dynamic_bitset REQUIRED)
+else()
+    set(BOOST_INCLUDE_LIBRARIES dynamic_bitset)
+    add_subdirectory(../../../.. deps/boost EXCLUDE_FROM_ALL)
+endif()
+
+add_executable(main main.cpp)
+target_link_libraries(main Boost::dynamic_bitset)
+
+enable_testing()
+add_test(NAME main COMMAND main)

test/cmake_test/main.cpp

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+#include <boost/dynamic_bitset.hpp>
+
+int main()
+{
+    const boost::dynamic_bitset<> db{3, 4};
+
+    return db[2] ? 0 : 1;
+}