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).

Author: bors

mk/reconfig.mk

@@ -15,7 +15,11 @@ rwildcard=$(foreach d,$(wildcard $1*),$(call rwildcard,$d/,$2) \
 
 ifndef CFG_DISABLE_MANAGE_SUBMODULES
 # This is a pretty expensive operation but I don't see any way to avoid it
-NEED_GIT_RECONFIG=$(shell cd "$(CFG_SRC_DIR)" && "$(CFG_GIT)" submodule status | grep -c '^\(+\|-\)')
+# NB: This only looks for '+' status (wrong commit checked out), not '-' status
+# (nothing checked out at all).  `./configure --{llvm,jemalloc,libuv}-root`
+# will explicitly deinitialize the corresponding submodules, and we don't
+# want to force constant rebuilds in that case.
+NEED_GIT_RECONFIG=$(shell cd "$(CFG_SRC_DIR)" && "$(CFG_GIT)" submodule status | grep -c '^+')
 else
 NEED_GIT_RECONFIG=0
 endif