muSpectre/cmake/Eigen3.cmake
0987ecf93faffeat/doc

Eigen3.cmake
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			download_external_project(Eigen3
			URL "https://bitbucket.org/eigen/eigen"
			TAG "${_Eigen3_version}"
			BACKEND HG
			THIRD_PARTY_SRC_DIR ${_Eigen3_external_dir}
			${_Eigen3_update}
			)

			file(READ "${_Eigen3_external_dir}/Eigen3/Eigen/src/Core/util/Macros.h" _eigen_version_header)
			string(REGEX MATCH "define[ \t]+EIGEN_WORLD_VERSION[ \t]+([0-9]+)" _eigen_world_version_match "${_eigen_version_header}")
			set(_eigen_world_version "${CMAKE_MATCH_1}")
			string(REGEX MATCH "define[ \t]+EIGEN_MAJOR_VERSION[ \t]+([0-9]+)" _eigen_major_version_match "${_eigen_version_header}")
			set(_eigen_major_version "${CMAKE_MATCH_1}")
			string(REGEX MATCH "define[ \t]+EIGEN_MINOR_VERSION[ \t]+([0-9]+)" _eigen_minor_version_match "${_eigen_version_header}")
			set(_eigen_minor_version "${CMAKE_MATCH_1}")
			set(Eigen3_VERSION ${_eigen_world_version}.${_eigen_major_version}.${_eigen_minor_version})

			set(EIGEN3_INCLUDE_DIR ${_Eigen3_external_dir}/Eigen3 CACHE PATH "Eigen include directory")
			add_library(eigen3 INTERFACE)
			add_library(Eigen3::Eigen ALIAS eigen3)
			target_include_directories(eigen3 SYSTEM INTERFACE
			$<BUILD_INTERFACE:${EIGEN3_INCLUDE_DIR}>)

			set(Eigen3_FOUND TRUE CACHE BOOL INTERNAL "")
			set(Eigen3_FOUND_EXTERNAL TRUE CACHE BOOL INTERNAL "")
			message(STATUS "Eigen3 ${Eigen3_VERSION}")