support manually set STDFACE_DIR (originally from hphi)

CMakeLists.txt

@@ -113,21 +113,30 @@ endif()
 # ref: https://cliutils.gitlab.io/modern-cmake/chapters/projects/submodule.html
 set(MVMC ON) # Option for upstream StdFace.
 find_package(Git QUIET)
-set(STDFACE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/src/StdFace")
-if(GIT_FOUND AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/.git")
-  # Update submodules as needed
-  if(GIT_SUBMODULE_UPDATE)
-    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 ${GIT_SUBMOD_RESULT}, please checkout submodules")
+
+if(NOT STDFACE_DIR)
+  set(STDFACE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/src/StdFace")
+  if(GIT_FOUND AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/.git")
+    # Update submodules as needed
+    if(GIT_SUBMODULE_UPDATE)
+      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 "${STDFACE_DIR}/CMakeLists.txt")
+      message(FATAL_ERROR "The submodule StdFace were not downloaded! GIT_SUBMODULE_UPDATE was turned off or failed. Please update submodules and try again.")
+    endif()
+  endif()
+else()
   if(NOT EXISTS "${STDFACE_DIR}/CMakeLists.txt")
-    message(FATAL_ERROR "The submodule StdFace were not downloaded! GIT_SUBMODULE_UPDATE was turned off or failed. Please update submodules and try again.")
+    message(FATAL_ERROR "STDFACE_DIR is manually set to ${STDFACE_DIR}, but ${STDFACE_DIR}/CMakeLists.txt does not exists")
+  else()
+    message(STATUS "STDFACE_DIR is manually set to ${STDFACE_DIR}.")
   endif()
 endif()
 add_subdirectory("${STDFACE_DIR}")