Use '$SED' instead of 'sed'

At least on Solaris berkeley-db.m4  fails to extract the correct db version,
since it doesn't use the stream editor, which got selected by 'configure' and
friends. Instead it uses hardcoded 'sed' which leads to SVN_DB_LIBS='-ldb-' and
is obviously wrong.

Suggested patch follows.

Original issue reported by elkner