configure: replace echo "" with plain echo

When no arguments are given, echo prints a blank line.  These two are
equivalent:

  $ echo ""
  $ echo

Replace the former by the latter.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
auto merge of #6970 : artagnon/rust/configure, r=catamorphism

When no arguments are given, echo prints a blank line.  These two are
equivalent:

  $ echo ""
  $ echo

Replace the former by the latter.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>