create and validate build_variables.bzl (#8326)

First step of #8268 -- we are moving from buck-extracted filelist to
using one shared filelist, and the first step is to create the shared
filelist and validate it against the buck generation.

Differential Revision:
[D80187441](https://our.internmc.facebook.com/intern/diff/D80187441)

Author: swolchok

CMakeLists.txt

@@ -50,7 +50,10 @@
 cmake_minimum_required(VERSION 3.29)
 project(executorch)
 
+set(EXECUTORCH_ROOT ${CMAKE_CURRENT_SOURCE_DIR})
+
 include(${PROJECT_SOURCE_DIR}/tools/cmake/common/preset.cmake)
+include(${PROJECT_SOURCE_DIR}/tools/cmake/Codegen.cmake)
 include(${PROJECT_SOURCE_DIR}/tools/cmake/Utils.cmake)
 include(CMakeDependentOption)
 include(ExternalProject)
@@ -123,8 +126,6 @@ set(CMAKE_INSTALL_RPATH_USE_LINK_PATH ON)
 # Instead please use `find_package(executorch REQUIRED)` in the example
 # directory and add a new executable in the example `CMakeLists.txt`.
 
-set(EXECUTORCH_ROOT ${CMAKE_CURRENT_SOURCE_DIR})
-
 if(NOT EXECUTORCH_ENABLE_LOGGING)
   # Avoid pulling in the logging strings, which can be large. Note that this
   # will set the compiler flag for all targets in this directory, and for all
@@ -320,6 +321,7 @@ if(NOT EXECUTORCH_SRCS_FILE)
   message(STATUS "executorch: Generating source lists")
   set(EXECUTORCH_SRCS_FILE "${CMAKE_CURRENT_BINARY_DIR}/executorch_srcs.cmake")
   extract_sources(${EXECUTORCH_SRCS_FILE})
+  executorch_validate_build_variables()
 endif()
 
 # This file defines the `_<target>__srcs` variables used below.