Weird LLVM build problem

I posted about this before, but I have more information now.
I have two machines:
  a MacPro with Xcode 3.2.5 installed - LLVM builds fine there.
  a MacBookPro with Xcode 4.0.1 installed - LLVM fails to build there.

In both cases, the LLVM code base is not on the boot volume, and I am building out-of-tree.
The directory hierarchy looks like this:
  llvm/
  llvm-build/
  llvm-nodebug/

for a clean build, I do:
  rm -rf llvm-build
  mkdir llvm-build ; cd llvm-build
  ../llvm/configure
  make

make -k -n shows (in the failing build):

make -k -n
for dir in lib/Support utils lib/VMCore lib tools/llvm-config tools runtime docs unittests; do \
    if ([ ! -f $dir/Makefile ] || \
        command test $dir/Makefile -ot /$dir/Makefile ); then \
      /Volumes/EyeFive/Marshall/Sources/LLVM/llvm/autoconf/mkinstalldirs $dir; \
      /bin/cp /$dir/Makefile $dir/Makefile; \
    fi; \
    (make -C $dir all ) || exit 1; \
  done

while in the succeeding build:

for dir in lib/Support utils lib/VMCore lib tools/llvm-config tools runtime docs unittests; do \
    if ([ ! -f $dir/Makefile ] || \
        command test $dir/Makefile -ot /Volumes/Bay2/LLVM/llvm/$dir/Makefile ); then \
      /Volumes/Bay2/LLVM/llvm/autoconf/mkinstalldirs $dir; \
      /bin/cp /Volumes/Bay2/LLVM/llvm/$dir/Makefile $dir/Makefile; \
    fi; \
    (make -C $dir all ) || exit 1; \
  done

It appears that somehow the variable LLVM_SRC_ROOT is somehow getting set to the empty string.

Here's what I think is the appropriate bit from llvm/Makefile.rules:

ifneq ($(PROJ_SRC_ROOT),$(PROJ_OBJ_ROOT))
$(RecursiveTargets)::
  $(Verb) for dir in $(DIRS); do \
    if ([ ! -f $$dir/Makefile ] || \
        command test $$dir/Makefile -ot $(PROJ_SRC_DIR)/$$dir/Makefile ); then \
      $(MKDIR) $$dir; \
      $(CP) $(PROJ_SRC_DIR)/$$dir/Makefile $$dir/Makefile; \
    fi; \
    ($(MAKE) -C $$dir $@ ) || exit 1; \
  done
else
$(RecursiveTargets)::
  $(Verb) for dir in $(DIRS); do \
    ($(MAKE) -C $$dir $@ ) || exit 1; \
  done
endif

Any ideas?
I've tried a clean checkout of the LLVM sources - no change.

Both machines are using gmake 3.8.1.

Thanks in advance!

-- Marshall

Marshall Clow Idio Software <mailto:mclow.lists@gmail.com>

A.D. 1517: Martin Luther nails his 95 Theses to the church door and is promptly moderated down to (-1, Flamebait).
        -- Yu Suzuki