Update cmake submodule handling

- Guard against duplicating btwxt builds

cmake/initialize-submodules.cmake

@@ -0,0 +1,27 @@
+macro(initialize_submodules)
+  if(GIT_FOUND AND EXISTS "${PROJECT_SOURCE_DIR}/.git")
+    set(git_modules_file "${PROJECT_SOURCE_DIR}/.gitmodules")
+    if (EXISTS ${git_modules_file})
+      file(STRINGS ${git_modules_file} file_lines)
+      foreach(line ${file_lines})
+        if (${line} MATCHES "url =")
+          string(REGEX REPLACE "\\s*url = .*/(.*).git" "\\1" submodule "${line}")
+          string(STRIP "${submodule}" submodule)
+          if(NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/${submodule}")
+            message(FATAL_ERROR "Submodule directory \"${CMAKE_CURRENT_SOURCE_DIR}/${submodule}\" does not exist")
+          endif()
+          # Initialize submodule if it hasn't already been cloned
+          if(NOT EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/${submodule}/.git")
+            message(STATUS "Initialize ${submodule} submodule")
+            execute_process(COMMAND ${GIT_EXECUTABLE} submodule update --init --recursive "${CMAKE_CURRENT_SOURCE_DIR}/${submodule}"
+              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 --recursive ${CMAKE_CURRENT_SOURCE_DIR}/${submodule} failed with ${GIT_SUBMOD_RESULT}, please checkout submodules")
+            endif()
+          endif()
+        endif()
+      endforeach()
+    endif()
+  endif()
+endmacro()

vendor/CMakeLists.txt

@@ -1,17 +1,8 @@
 
-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 --recursive failed with ${GIT_SUBMOD_RESULT}, please checkout submodules")
-        endif()
-    endif()
-endif()
 
+include(initialize-submodules)
+initialize_submodules()
 
-add_subdirectory(btwxt)
+if (NOT TARGET btwxt)
+  add_subdirectory(btwxt)
+endif ()