Building clang with MSYS under Windows

I follow the instructions for building clang with MSYS under Windows given on the clang web site.

The 'configure' command runs to completion without any problems.

When I invoke 'make' I get:

'Makefile:139: /Makefile.rules: No such file or directory
make: *** No rule to make target `/Makefile.rules'. Stop.'

In the generated at line 139 I see:

'include $(LLVM_SRC_ROOT)/Makefile.rules'

What has gone wrong ?

LLVM_SRC_ROOT was not set correctly. Run "make -p" and check the
output to see whether LLVM_SRC_ROOT is undefined or maybe defined as
empty. (Redirect the output or pipe into 'less' for easier handling).