Use fetchcontent for eigen, again

CMakeLists.txt

@@ -216,18 +216,18 @@ find_package(ZLIB REQUIRED)
 
 find_package(Eigen3 QUIET)
 
-if(Eigen3_FOUND)
-	get_target_property(EIGEN_INCLUDE_DIR Eigen3::Eigen INTERFACE_INCLUDE_DIRECTORIES)
-else()
-	ExternalProject_Add(
-		local_Eigen3
+if(NOT Eigen3_FOUND)
+	FetchContent_Declare(
+		Eigen3
 		GIT_REPOSITORY https://gitlab.com/libeigen/eigen.git
 		GIT_TAG 3.4.0
-		CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${CMAKE_CURRENT_BINARY_DIR}/external
 	)
 
-	set(EIGEN3_D local_Eigen3)
-	set(EIGEN_INCLUDE_DIR ${CMAKE_CURRENT_BINARY_DIR}/external/include/eigen3)
+	FetchContent_GetProperties(Eigen3)
+	if(NOT Eigen3_POPULATED)
+	  FetchContent_Populate(Eigen3)
+	  add_subdirectory(${Eigen3_SOURCE_DIR} ${Eigen3_BINARY_DIR} EXCLUDE_FROM_ALL)
+	endif()
 endif()
 
 include(FindFilesystem)
@@ -338,15 +338,12 @@ target_include_directories(cifpp
 	"$<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>"
 	"$<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>"
 	PRIVATE
-	"${EIGEN_INCLUDE_DIR}"
 	"${BOOST_REGEX_INCLUDE_DIR}"
 )
 
-target_link_libraries(cifpp PUBLIC Threads::Threads ZLIB::ZLIB ${CIFPP_REQUIRED_LIBRARIES})
-
-if(${EIGEN3_D})
-	add_dependencies(cifpp ${EIGEN3_D})
-endif()
+target_link_libraries(cifpp
+	PUBLIC Threads::Threads ZLIB::ZLIB ${CIFPP_REQUIRED_LIBRARIES}
+	PRIVATE Eigen3::Eigen)
 
 if(CMAKE_CXX_COMPILER_ID STREQUAL "AppleClang")
 	target_link_options(cifpp PRIVATE -undefined dynamic_lookup)
@@ -542,8 +539,7 @@ if(BUILD_TESTING)
 
 		add_executable(${CIFPP_TEST} ${CIFPP_TEST_SOURCE} "${CMAKE_CURRENT_SOURCE_DIR}/test/test-main.cpp")
 
-		target_link_libraries(${CIFPP_TEST} PRIVATE Threads::Threads cifpp::cifpp Catch2::Catch2)
-		target_include_directories(${CIFPP_TEST} PRIVATE ${EIGEN_INCLUDE_DIR})
+		target_link_libraries(${CIFPP_TEST} PRIVATE Threads::Threads cifpp::cifpp Catch2::Catch2 Eigen3::Eigen)
 
 		if(MSVC)
 			# Specify unwind semantics so that MSVC knowns how to handle exceptions