Revert "Temporarily revert "build: avoid hardcoding the libxml2 library name""

This reverts commit 2e75681.  Restore
the clean up change.  The underlying CMake issue was resolved in
372ad32.

Author: compnerd

llvm/cmake/config-ix.cmake

@@ -166,7 +166,6 @@ if(NOT LLVM_USE_SANITIZER MATCHES "Memory.*")
         else()
           include_directories(${LIBXML2_INCLUDE_DIR})
         endif()
-        set(LIBXML2_LIBS "xml2")
       endif()
     endif()
   endif()

llvm/lib/WindowsManifest/CMakeLists.txt

@@ -1,18 +1,12 @@
-set(system_libs)
-if( CMAKE_HOST_UNIX )
-  if( LLVM_LIBXML2_ENABLED )
-    set(system_libs ${system_libs} ${LIBXML2_LIBS})
-  endif()
-endif()
-
 add_llvm_component_library(LLVMWindowsManifest
   WindowsManifestMerger.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/WindowsManifest
-  ${Backtrace_INCLUDE_DIRS}
+  ${Backtrace_INCLUDE_DIRS})
 
-  LINK_LIBS ${system_libs}
-  )
-
-set_property(TARGET LLVMWindowsManifest PROPERTY LLVM_SYSTEM_LIBS "${system_libs}")
+if(LIBXML2_LIBRARIES)
+  target_link_libraries(LLVMWindowsManifest PUBLIC ${LIBXML2_LIBRARIES})
+  set_property(TARGET LLVMWindowsManifest PROPERTY
+    LLVM_SYSTEM_LIBS ${LIBXML2_LIBRARIES})
+endif()