Liboverlay-scrollbar package build creates a duplicate library file

I am rebuilding packages for the Unity-for-Arch project on Arch Linux. While I was upgrading overlay-scrollbar to the Xenial package version, I encountered a problem installing liboverlay-scrollbar3. When I try to install it on my system, it complained about a library file already existing on my system. /lib/gtk-2/modules/liboverlay-scrollbar.so was the library it complained about, so I did a little digging into the source file, and I found the following problem. In all the build-gtk3 folder's makefiles, I found this line:

GTK_MODULES_DIR = ${exec_prefix}/lib/gtk-2.0/modules

It appears in every Makefile in the build-gtk3 directory. I looked in my previous version's (vivid) Makefiles, and I found that in each location, this appears instead:

GTK_MODULES_DIR = ${exec_prefix}/lib/gtk-3.0/modules

Since the package is supposed to be build against gtk-3.0 and should install into a gtk-3.0 location, I patched the source Makefiles and re-compiled the packages. As soon as I did this, the complaint about the library file already existing vanished. Should I report this as a bug?