[Bro] Issues with DESTDIR support

[Siwek, Jonathan Luke]

> Most of bro's build system supports DESTDIR correctly; lib/, share/, and
> bin/ all get populated properly under the sandbox directory.  But, it
> seems that all config-file-writing code does not. 

You're right, the InstallPackageConfigFile.cmake script does extra precautionary checks so that user-modified config files don't get clobbered at install time, but, in doing that, it worked around the normal file install process and ends up not respecting DESTDIR.

If you can work from our git repository, I made a change on the "topic/jsiwek/destdir-fix" branch of our "cmake" repository that looks like it fixes the problem.  To test it out, you'd do something like:

    git clone --recursive git://bro-ids.org/bro
    cd bro
    for repo in `find -d . -name cmake`; do (cd $repo && git fetch && git checkout topic/jsiwek/destdir-fix); done

And then the usual configure, make, install stuff.  Let me know if it works for you and I'll get it merged into the master branch.

+Jon