- workaround for https://github.com/openmc-dev/openmc/issues/1891
- fix xtl,xtensor dependencies

--- CMakeLists.txt.orig	2021-06-14 14:22:01 UTC
+++ CMakeLists.txt
@@ -174,7 +174,7 @@ if(GIT_FOUND AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/.
 endif()
 
 # Check to see if submodules exist (by checking one)
-if(NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/vendor/pugixml/CMakeLists.txt")
+if(FALSE AND NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/vendor/pugixml/CMakeLists.txt")
   message(FATAL_ERROR "The git submodules were not downloaded! GIT_SUBMODULE was \
     turned off or failed. Please update submodules and try again.")
 endif()
@@ -206,9 +206,10 @@ if (NOT (CMAKE_VERSION VERSION_LESS 3.13))
   cmake_policy(SET CMP0079 NEW)
 endif()
 
-add_subdirectory(vendor/xtl)
-set(xtl_DIR ${CMAKE_CURRENT_BINARY_DIR}/vendor/xtl)
-add_subdirectory(vendor/xtensor)
+#add_subdirectory(vendor/xtl)
+#set(xtl_DIR ${CMAKE_CURRENT_BINARY_DIR}/vendor/xtl)
+#add_subdirectory(vendor/xtensor)
+find_package(xtensor REQUIRED)
 
 #===============================================================================
 # GSL header-only library