I recently had the same issue. For various reasons (IT; lazy) I didn’t have access to github/Required* files.
My windows sdk (Windows Kits/8.1/…) for whatever reason did not have the metahost.h file - and who knows what else.
Reinstalling/repairing the sdk and/or visual studio 2013 in place did not fix the issue.
I took the mini-nuke option, uninstalled all versions of .net and visual studio (2010,2012, 2013), and reinstalled vs2013 (which includes winsdk 8.1 and dotnet 4.5)
metahost.h lives, and UE4 compiles again. I did not have to do anything to the projects or unreal itself.