Use automake’s silent-rules mode if available, for quieter build output

With automake 1.11, this gives us Linux kernel–style silent output
that makes warnings more obvious:

  GEN    userinput.pb.cc
  CXX    userinput.pb.o
  AR     libmoshprotos.a
  CXXLD  mosh-client

(Use ‘make V=1’ for the traditional verbose output.)

Signed-off-by: Anders Kaseorg <andersk@mit.edu>