Support building the NEWS file in out-of-source-tree builds.

Copy the built file into the source directory (since it's generated but checked
in).  Don't fail to build, though, if we can't do that copy.

Change-Id: Ia94a19b6f813ee78b191cd09d51198462f95b223
Reviewed-on: https://code.wireshark.org/review/3425
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>