Add a check for project submodules.

CMakeLists.txt

@@ -77,6 +77,29 @@ endmacro()
 # Save the old value of CMAKE_REQUIRED_FLAGS
 set( TEMP_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS} )
 
+# Check submodules
+if(NOT MANUAL_SUBMODULES)
+  find_package(Git)
+  if(GIT_FOUND)
+    function (check_submodule relative_path)
+      execute_process(COMMAND git rev-parse "HEAD" WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/${relative_path} OUTPUT_VARIABLE localHead)
+      execute_process(COMMAND git rev-parse "HEAD:${relative_path}" WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} OUTPUT_VARIABLE checkedHead)
+      string(COMPARE EQUAL "${localHead}" "${checkedHead}" upToDate)
+      if (upToDate)
+        message(STATUS "Submodule '${relative_path}' is up-to-date")
+      else()
+        message(FATAL_ERROR "Submodule '${relative_path}' is not up-to-date. Please update all submodules with\ngit submodule update --init --recursive --force\nor run cmake with -DMANUAL_SUBMODULES=1\n")
+      endif()
+    endfunction ()
+
+    message(STATUS "Checking submodules")
+    check_submodule(docs)
+    check_submodule(libraries/fc)
+  endif()
+endif()
+# Make sure to always re-run the test.
+unset( MANUAL_SUBMODULES CACHE )
+
 # Fortify source
 if (CMAKE_COMPILER_IS_GNUCXX)
 	if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")