build: avoid hardcoding the libxml2 library name

FindLibXml2 will set the LIBXML2_LIBRARIES variable to the libraries that
we must link against. This will be an empty string if libxml2 is not
found. Avoid hardcoding the library name as xml2 in the configuration.
Simplify the usage in the WindowsManifest library.

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}
 
-  LINK_LIBS ${system_libs}
+  LINK_LIBS ${LIBXML2_LIBRARIES}
   )
 
-set_property(TARGET LLVMWindowsManifest PROPERTY LLVM_SYSTEM_LIBS "${system_libs}")
+set_property(TARGET LLVMWindowsManifest PROPERTY
+  LLVM_SYSTEM_LIBS ${LIBXML2_LIBRARIES})