In the UNIX build, make some compiler warnings which we usually fix up
fatal build errors. This should help with catching these warnings on the
buildbots.

* configure.ac
  (CMAINTAINERFLAGS): Make -Wshadow, -Wunused, and -Wmissing-declarations
   fatal build errors.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1441110 13f79535-47bb-0310-9956-ffa450edef68