By putting together various pieces from the previous answers I was finally able to get a matching .Net Framework 4.6 and the Targeting Pack for it, and once both were installed the v4.6 Framework shows up as an available target framework Visual Studio 2012.
I downloaded the standalone installer for Framework 4.6 (NDP46-KB3045557-x86-x64-AllOS-ENU.exe) from http://www.microsoft.com/en-us/download/details.aspx?id=48130, and installed it first.
Then I downloaded and mounted the ISO for Visual Studio 2015 Professional ISO from https://www.visualstudio.com/downloads/download-visual-studio-vs, browsed to the netfx_46mtpack subdirectory, and installed netfx_46mtpack.msi.
