make distclean fails if libm_machine_dir is empty

[Patrick Mansfield]

Hi -

Running with current cvs, distclean" fails if libm_machine_dir is not set
(really when it is an empty string).

Failure output is:

Making distclean in machine
/bin/sh: line 0: cd: machine: No such file or directory

See newlib/libm/Makefile.am and newlib/libm/Makefile.in, and look for
"machines".

What sets HAVE_LIBM_MACHINE_DIR?

Any fixes?

-- Patrick Mansfield