premake5: Disallow Microsoft toolsets older than Visual Studio 2013.

We only explicitly allow toolsets that are tested and compiled against - here it's Visual Studio 2013 and 2015.

premake5.lua

@@ -55,9 +55,15 @@ solution "iw4x"
 		language "C++"
 		files { "./src/**.hpp", "./src/**.cpp" }
 		includedirs { "%{prj.location}" }
-		toolset "msc" -- only support Microsoft's compiler
 		architecture "x32"
 
+		-- Allow newer Microsoft toolsets but fall back to VS2013
+		if _ACTION == "vs2015" then
+			toolset "msc-140"
+		else
+			toolset "msc-120"
+		end
+
 		configuration "Normal"
 			defines { "NDEBUG" }
 			flags { "MultiProcessorCompile", "Symbols" }