libomptarget: Disable on Win32

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

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

llvm-svn: 294758
GitOrigin-RevId: 1155ad1bc816a78ab1dfd70b8dadf8e7b3c47f48
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 35b6f6c..b8662ba 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -3,4 +3,7 @@
 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()