Trouble creating library package

The Makefile you are using is not respecting the DESTDIR variable - packages should never install directly into /usr at build time, but instead put files into a temporary tree. You've mentioned that you added the Makefile yourself - the usual case is that you'd need to make it use the DESTDIR variable as a prefix for all install paths. Having looked at the package build instructions, the package uses scons as a build system, which is a replacement of sorts for make. dh_make most likely doesn't have a template for building packages with this, so you should look for packages that do use it for building, such as yafray, and look at their debian/rules

As package builds are never done as root, it is better to have this error shown & corrected rather than having a package overwrite important files as the package is being built on an unsuspecting developer's system.