Add headers to generated CMake targets

[Flamefire]

For header-only libraries this adds a target to IDEs containing the headers (CMake 3.19+) For compiled libraries it includes the headers in the existing target

E.g. for optional it outputs:

# Generated by `boostdep --cmake optional`
# Copyright 2020, 2021 Peter Dimov
# Distributed under the Boost Software License, Version 1.0.
# https://www.boost.org/LICENSE_1_0.txt

cmake_minimum_required(VERSION 3.8...3.20)

project(boost_optional VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)

add_library(boost_optional INTERFACE)
add_library(Boost::optional ALIAS boost_optional)

target_include_directories(boost_optional INTERFACE include)
if(NOT CMAKE_VERSION VERSION_LESS "3.19")
  file(GLOB_RECURSE headers include/*.hpp)
  target_sources(boost_optional PRIVATE ${headers})
endif()

target_link_libraries(boost_optional
  INTERFACE
    Boost::assert
    Boost::config
    Boost::core
    Boost::move
    Boost::static_assert
    Boost::throw_exception
    Boost::type_traits
)

target_compile_features(boost_optional INTERFACE cxx_std_11)

if(BUILD_TESTING AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/test/CMakeLists.txt")

  add_subdirectory(test)

endif()