[Build] Use LIBXML2_LIBRARIES from find_package

[andrewrk]

Summary

Previously we pass the string "xml2" to the link libraries of the WindowsManifest library. This prevents static linking against libxml2.

This patch fixes the build to follow the cmake convention of using the output of find_package as the LINK_LIBS argument.

[labath]

I think you'll run into issues if you try to land this patch. I tried to do the same thing with zlib (see r319751 and related commits), but I had to roll it back because it broke things for a lot of people. Basically, the issue is we have code (e.g., llvm-config --system-libs) which assumes you can just paste "-l" in front of the system_libs libraries and get a valid linker line. This works only if system_libs is a list of bare lib names (and not a full file path).

I wish we had a better story for controlling the link behavior (I'm particularly interested in forcing static against a static library, when both static&dynamic libs are present), but right now I don't see how we could achieve that (without a big refactor).

[labath]

Hmm... I see now that you add this only to the library list of the WindowsManifest module. I this case you might be able to get away with it because it will not make it's way into the llvm-config --system-libs output. However, this may actually be a sign of a bigger problem, as that may mean that things (external projects, mostly) which link against this library will not get the complete link command.

llvm/cmake/config-ix.cmake

if (NOT PURE_WINDOWS)
     find_library(ICONV_LIBRARY_PATH NAMES iconv libiconv libiconv-2 c)
     set(LLVM_LIBXML2_ENABLED 0)
     set(LIBXML2_FOUND 0)
     if((LLVM_ENABLE_LIBXML2) AND ((CMAKE_SYSTEM_NAME MATCHES "Linux") AND (ICONV_LIBRARY_PATH) OR APPLE))
       find_package(LibXml2)
       if (LIBXML2_FOUND)
         set(LLVM_LIBXML2_ENABLED 1)
         include_directories(${LIBXML2_INCLUDE_DIR})
-        set(LIBXML2_LIBS "xml2")
       endif()
     endif()
   endif()
 endif()
 
 if (LLVM_ENABLE_LIBXML2 STREQUAL "FORCE_ON" AND NOT LLVM_LIBXML2_ENABLED)
   message(FATAL_ERROR "Failed to congifure libxml2")
 endif()

llvm/lib/WindowsManifest/CMakeLists.txt

 set(system_libs)
 if( CMAKE_HOST_UNIX )
   if( LLVM_LIBXML2_ENABLED )
-    set(system_libs ${system_libs} ${LIBXML2_LIBS})
+    set(system_libs ${system_libs} ${LIBXML2_LIBRARIES})
   endif()
 endif()
 
 add_llvm_library(LLVMWindowsManifest
   WindowsManifestMerger.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${LLVM_MAIN_INCLUDE_DIR}/llvm/WindowsManifest
   ${Backtrace_INCLUDE_DIRS}
 
   LINK_LIBS ${system_libs}
   )
 
 set_property(TARGET LLVMWindowsManifest PROPERTY LLVM_SYSTEM_LIBS "${system_libs}")