Add 'FetchEigen.cmake'

FetchEigen.cmake

@@ -0,0 +1,22 @@
+# usage:
+# FetchContent_MakeAvailable(Eigen)
+# target_link_libraries(${PROJECT_NAME} PRIVATE Eigen3::Eigen)
+
+if (NOT DEFINED EIGEN_GIT_TAG)
+    message(FATAL_ERROR "EIGEN_GIT_TAG is not defined.\n see https://gitlab.com/libeigen/eigen and grab a release tag.\n example: set(EIGEN_GIT_TAG 3.4.0)")
+endif()
+
+FetchContent_Declare(
+        Eigen
+        GIT_REPOSITORY https://gitlab.com/libeigen/eigen.git
+        GIT_TAG ${EIGEN_GIT_TAG}
+        GIT_SHALLOW TRUE
+        GIT_PROGRESS TRUE
+)
+set(EIGEN_BUILD_DOC OFF)
+set(BUILD_TESTING OFF)
+set(CMAKE_Fortran_COMPILER_WORKS ON)
+set(EIGEN_BUILD_PKGCONFIG OFF)
+
+include(${CMAKE_SOURCE_DIR}/cmake/Utils.cmake)
+print_git_tag(${EIGEN_GIT_TAG})