message: build: bump version to 0.16.0
  Update version after branching off.
  Tree-SHA512:
diff: dnl require autoconf 2.60 (AS_ECHO/AS_ECHO_N)
  AC_PREREQ([2.60])
  define(_CLIENT_VERSION_MAJOR, 0)
  -define(_CLIENT_VERSION_MINOR, 15)
  -define(_CLIENT_VERSION_REVISION, 99)
  +define(_CLIENT_VERSION_MINOR, 16)
  +define(_CLIENT_VERSION_REVISION, 0)
  define(_CLIENT_VERSION_BUILD, 0)
  -define(_CLIENT_VERSION_IS_RELEASE, false)
  +define(_CLIENT_VERSION_IS_RELEASE, true)
  define(_COPYRIGHT_YEAR, 2018)
  define(_COPYRIGHT_HOLDERS,[The %s developers])
  define(_COPYRIGHT_HOLDERS_SUBSTITUTION,[[Bitcoin Core]])
  