Some packaging cleanup/fixes

I pulled the source of this package when I couldn't successfully build a project in CMake when using find_package(LEMON) despite having it installed. There are two patches to fix the way the build works with regard to the CMake config file: one to modify the config file post-build to put in $(DEB_HOST_MULTIARCH), and another to change where the CMake config file gets installed to (now in the multilib-tagged cmake dir like several other packages on my system).

To get it to build I needed to add a d-shlib override, not sure if I did this correctly or not since it's my first time using that tool.

I also looked upstream and found there were changes since the release that had gotten backported, so I cherry-picked the ones that looked relevant and easy merges (used gbp pq to adjust the patch series).

Hopefully this is useful (and a reasonable method for submission) and can get merged, at least the cmake and d-shlib changes, since at the moment anyway a CMake-using project on Stretch can't build against this without themselves including a find-module for this package, which somewhat defeats the purpose of it distributing a CMake config file.

Let me know if you have any questions!