Pick up submodules from CMake config

Author: DanShort12

CMakeLists.txt

@@ -18,6 +18,26 @@ set(OPENMC_DIR /opt/openmc)
 set(OPENMC_INC_DIR ${OPENMC_DIR}/include)
 set(OPENMC_LIB_DIR ${OPENMC_DIR}/lib)
 
+# Ensure submodules are available and up to date
+find_package(Git QUIET)
+if(GIT_FOUND AND EXISTS "${PROJECT_SOURCE_DIR}/.git")
+  # Update submodules as needed
+  option(GIT_SUBMODULE "Check submodules during build" ON)
+  if(GIT_SUBMODULE)
+    message(STATUS "Submodule update")
+    execute_process(COMMAND ${GIT_EXECUTABLE} submodule update --init --recursive
+                    WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
+                    RESULT_VARIABLE GIT_SUBMOD_RESULT)
+    if(NOT GIT_SUBMOD_RESULT EQUAL "0")
+      message(FATAL_ERROR "git submodule update --init failed with ${GIT_SUBMOD_RESULT}, please checkout submodules")
+    endif()
+  endif()
+endif()
+
+if(NOT EXISTS "${PROJECT_SOURCE_DIR}/pybind11/CMakeLists.txt")
+  message(FATAL_ERROR "The submodules were not downloaded! GIT_SUBMODULE was turned off or failed. Please update submodules and try again.")
+endif()
+
 # Build source_sampling
 list(APPEND source_sampling_SOURCES
   ${SRC_DIR}/source_sampling.cpp