cmake can take EIGEN_INCLUDE_DIR as command line param

Former-commit-id: 7357215e42098ab342bdc5ef94ede89a6e9cef6d

shared/cmake/CMakeLists.txt

@@ -65,7 +65,9 @@ set(LIBIGL_EXTERNAL "${LIBIGL_ROOT}/external")
 set(NANOGUI_DIR "${LIBIGL_EXTERNAL}/nanogui")
 
 ### Eigen ###
-set(EIGEN_INCLUDE_DIR "${NANOGUI_DIR}/ext/eigen")
+if(NOT EIGEN_INCLUDE_DIR)
+  set(EIGEN_INCLUDE_DIR "${NANOGUI_DIR}/ext/eigen")
+endif()
 list(APPEND LIBIGL_INCLUDE_DIRS "${EIGEN_INCLUDE_DIR}")
 
 macro(CompileIGL_Module module_dir prefix)