build: avoid cached literals being linked against

If the value of the LibXml2 search is cached, it can cause an errant
link against LIBXML2_LIBRARIES-NOTFOUND if libxml2 is not found. Add
a guard against this.  Should repair the build bots.

Author: compnerd

llvm/lib/WindowsManifest/CMakeLists.txt

@@ -3,10 +3,10 @@ add_llvm_component_library(LLVMWindowsManifest
 
   ADDITIONAL_HEADER_DIRS
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/WindowsManifest
-  ${Backtrace_INCLUDE_DIRS}
+  ${Backtrace_INCLUDE_DIRS})
 
-  LINK_LIBS ${LIBXML2_LIBRARIES}
-  )
-
-set_property(TARGET LLVMWindowsManifest PROPERTY
-  LLVM_SYSTEM_LIBS ${LIBXML2_LIBRARIES})
+if(LIBXML2_LIBRARIES)
+  target_link_libraries(LLVMWindowsManifest PUBLIC ${LIBXML2_LIBRARIES})
+  set_property(TARGET LLVMWindowsManifest PROPERTY
+    LLVM_SYSTEM_LIBS ${LIBXML2_LIBRARIES})
+endif()