Improve diagnostics for unavailable pre-compiled LLVM packages.

Fixes https://github.com/mono/CppSharp/issues/693.

build/scripts/LLVM.lua

@@ -128,6 +128,12 @@ local use_7zip = os.is("windows")
 local archive_ext = use_7zip and ".7z" or ".tar.xz"
 
 function download_llvm()
+  local toolset = get_toolset_configuration_name()
+  if toolset == "vs2012" or toolset == "vs2013" then
+    error("Pre-compiled LLVM packages for your VS version are not available.\n" ..
+          "Please upgrade to a newer VS version or compile LLVM manually.")
+  end
+
   local base = "https://dl.dropboxusercontent.com/u/194502/CppSharp/llvm/"
   local pkg_name = get_llvm_package_name()
   local archive = pkg_name .. archive_ext