libomptarget: Disable on Win32

It's not supported, and currently breaks the weekly LLVM snapshot
builds.

Differential Revision: https://reviews.llvm.org/D29801

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@294758 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -3,4 +3,7 @@ cmake_minimum_required(VERSION 2.8 FATAL_ERROR)
 set(OPENMP_LLVM_TOOLS_DIR "" CACHE PATH "Path to LLVM tools for testing")
 
 add_subdirectory(runtime)
-add_subdirectory(libomptarget)
+
+if (NOT WIN32)
+  add_subdirectory(libomptarget)
+endif()