ignore uninitialized submodules when checking if ./configure should be re-run
auto merge of #17129 : epdtry/rust/misc/llvm-root-reconfig, r=brson

Currently `./configure --llvm-root=...` and similar flags will break incremental builds by forcing reconfiguration on every `make`.  This happens because `reconfig.mk` incorrectly treats submodules in the `-` (uninitialized) state as requiring reconfiguration, and `./configure` deliberately deinitializes unneeded submodules.  The fix is to reconfigure only when submodules are in the `+` state (wrong commit checked out).