[CMake] Don't build the libclang tests unless you're building libclang

This fixes a build issue reported by users at Apple.

llvm-svn: 254797
diff --git a/clang/unittests/CMakeLists.txt b/clang/unittests/CMakeLists.txt
index 6636d82..b85ec7e 100644
--- a/clang/unittests/CMakeLists.txt
+++ b/clang/unittests/CMakeLists.txt
@@ -25,6 +25,6 @@
 add_subdirectory(CodeGen)
 # FIXME: libclang unit tests are disabled on Windows due
 # to failures, mostly in libclang.VirtualFileOverlay_*.
-if(NOT WIN32) 
+if(NOT WIN32 AND CLANG_TOOL_LIBCLANG_BUILD) 
   add_subdirectory(libclang)
 endif()