So I managed to get gtkmm working on Visual Studio 2015, the instructions are super unclear so here's what I did:
1- Download the x64 or x86 release from (do not follow the compilation instructions) https://github.com/codekiddy2/Visual-Studio-gtkmm
Releases: https://github.com/codekiddy2/Visual-Studio-gtkmm/releases
2- Save and extract everything... somewhere, like in the root of C:\