Prevent premature @AR@ replacement in a sed expression.

Prevent premature @AR@ replacement in a sed expression.

The Makefile rule for make-debug-archive has a sed expression to replace
@AR@ will the installed name, but this was itself getting replaced when
the Makefile was configured, for a pattern like "s,ar,/path/prefix-,g".
Havoc ensued in the resulting make-debug-archive.

The fix matches it using a regex bracket expression, "[@]AR[@]", so sed
will still match it, but it's immune to configure's replacement.

Signed-off-by: Josh Stone <jistone@redhat.com>