I don't like to answer my own question on here, but since we were looking in the wrong direction, here goes:
The problem that I was experiencing where the compiler was running into 'undefined reference' errors was caused by the manually applied namespace 'Gtk' before SourceView at the top of the EditorWindow class. Due to the nature of the Vala compiler, it did not throw an error because SourceView is under the Gtk namespace, but apparently something went wrong between valac and gcc. As soon as I changed Gtk.SourceView
to SourceView
, the program compiled.