When COMPUTE_HEADER_DEPENDENCIES is set to "auto" and the compiler
does not support it, $(dep_dirs) becomes empty. "make clean" runs
"rm -rf $(dep_dirs)", which can fail in such a case.
Signed-off-by: Matt Kraai <matt.kraai@amo.abbott.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>