Visual Studio 2013 and .NET 4.6

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.

You need to install the .NET Framework 4.6 Targeting Pack.

The .NET Framework 4.6 Targeting Pack is a package that enables developers to build applications targeting the .NET Framework 4.6 using either Visual Studio 2013, Visual Studio 2012 or third party IDEs. You need to download and install the .NET Framework 4.6 prior to installing the targeting pack.

Referring to this support article by Microsoft:
Microsoft .NET Framework 4.6 Targeting Pack and Language Packs for Windows

In short you need three components:

1. The .Net 4.6 Framework itself of course
2. The .NET Framework 4.6 Targeting Pack
   • language neutral reference assemblies to compile against
   • download file name like “NDP46-TargetingPack-KB3045566.exe”
3. optional: The .NET Framework 4.6 Targeting Pack Language Packs
   • translated IntelliSense help files
   • download file name like “NDP46-TargetingPack-KB3045566-ENU.exe”
     (suffix -ENU for english, -DEU for german etc.)