[llvm] Fix incorrect usage of LIBXML2_INCLUDE_DIRS in the Windows release script

[aganea]

Before this fix, when building the Windows LLVM package with the latest cmake 3.29.3 I was seeing:

C:\git\llvm-project>llvm\utils\release\build_llvm_release.bat --version 19.0.0 --x64 --skip-checkout --local-python
...
-- Looking for FE_INEXACT
-- Looking for FE_INEXACT - found
-- Performing Test HAVE_BUILTIN_THREAD_POINTER
-- Performing Test HAVE_BUILTIN_THREAD_POINTER - Failed
-- Looking for mach/mach.h
-- Looking for mach/mach.h - not found
-- Looking for CrashReporterClient.h
-- Looking for CrashReporterClient.h - not found
-- Looking for pfm_initialize in pfm
-- Looking for pfm_initialize in pfm - not found
-- Could NOT find ZLIB (missing: ZLIB_LIBRARY ZLIB_INCLUDE_DIR)
CMake Error at C:/Program Files/CMake/share/cmake-3.29/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find LibXml2 (missing: LIBXML2_INCLUDE_DIR)
Call Stack (most recent call first):
  C:/Program Files/CMake/share/cmake-3.29/Modules/FindPackageHandleStandardArgs.cmake:600 (_FPHSA_FAILURE_MESSAGE)
  C:/Program Files/CMake/share/cmake-3.29/Modules/FindLibXml2.cmake:108 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  cmake/config-ix.cmake:167 (find_package)
  CMakeLists.txt:921 (include)


-- Configuring incomplete, errors occurred!

It looks like LIBXML2_INCLUDE_DIRS (with the extra 'S') is a result variable that is set by cmake after a call to find_package(LibXml2). It is actually LIBXML2_INCLUDE_DIR (without the 'S') that shold be used as a input before the find_package call, since the 'S' variable is unconditionally overwritten, see https://github.com/Kitware/CMake/blob/master/Modules/FindLibXml2.cmake#L96. I am unsure exactly why that worked with older cmake versions.

[aganea]

@CarlosAlbertoEnciso Can you please take a look? It was this comment https://reviews.llvm.org/D129571#3651781 that triggered the change to LIBXML2_INCLUDE_DIRS.

[CarlosAlbertoEnciso]

Looking at those original changes:

cmake -GNinja %cmake_flags% -DPYTHON_HOME=%python64_dir% -DPython3_ROOT_DIR=%python64_dir% -DLIBXML2_INCLUDE_DIR=%libxmldir%/include/libxml2 -DLIBXML2_LIBRARIES=%libxmldir%/lib/libxml2s.lib ..\llvm-project\llvm || exit /b

It is clear that the LIBXML2_INCLUDE_DIR is being set to the %libxmldir%/include/libxml2, which is what the patch is setting.

[CarlosAlbertoEnciso]

LGTM

[zmodem]

lgtm