Add a Boost-friendly subproject case to CMakeLists (#614)

CMakeLists.txt

@@ -1,15 +1,50 @@
 #
 # Copyright (c) 2017-2019 Mateusz Loskot <mateusz at loskot dot net>
+# Copyright (c) 2020-2021 Peter Dimov
 #
 # 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)
 #
+
+if(NOT CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)
+
+# Generated by `boostdep --cmake gil`
+
+cmake_minimum_required(VERSION 3.8...3.20)
+
+project(boost_gil VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)
+
+add_library(boost_gil INTERFACE)
+add_library(Boost::gil ALIAS boost_gil)
+
+target_include_directories(boost_gil INTERFACE include)
+
+target_link_libraries(boost_gil
+  INTERFACE
+    Boost::assert
+    Boost::concept_check
+    Boost::config
+    Boost::container_hash
+    Boost::core
+    Boost::filesystem
+    Boost::integer
+    Boost::iterator
+    Boost::mp11
+    Boost::preprocessor
+    Boost::type_traits
+    Boost::variant2
+)
+
+target_compile_features(boost_gil INTERFACE cxx_std_11)
+
+else()
+
 # **WARNING:**
 # The CMake configuration is only provided for convenience
 # of contributors. It does not export or install any targets,
 # deploy config files or support subproject workflow.
-#
+
 cmake_minimum_required(VERSION 3.10)
 
 #-----------------------------------------------------------------------------
@@ -250,3 +285,5 @@ endif()
 if(BOOST_GIL_BUILD_EXAMPLES AND BOOST_GIL_ENABLE_EXT_IO)
   add_subdirectory(example)
 endif()
+
+endif()