Makefile: fix shell quoting

Makefile: fix shell quoting

Makefile records paths to a few programs in GIT-BUILD-OPTIONS file.  These
paths need to be quoted twice: once to protect specials from the shell
that runs the generated GIT-BUILD-OPTIONS file, and again to protect them
(and the first level of quoting itself) from the shell that runs the
"echo" inside the Makefile.

You can test this by trying:

    $ ln -s /bin/tar "$HOME/Tes' program/tar"
    $ make TAR="$HOME/Tes' program/tar" test

Signed-off-by: Junio C Hamano <gitster@pobox.com>