[libxml2] Add link libraries for target LibXml2::LibXml2

[LilyWangL]

Describe the pull request

• What does your PR fix? Fixes libxml2 vcpkg-cmake-wrapper.cmake bugs #14468
  Target LibXml2::LibXml2 is not linked against all necessary libraries in vcpkg-cmake-wrapper.cmake. Target LibXml2::LibXml2 should link with Iconv, Charset, lzma, zlib, ws2_32 (windows only) libraries.

[jmelas]

I think still LIBXML2_LIBRARIES is not populated correctly.
The missing libraries should first be added to LIBXML2_LIBRARIES variable
and then target LibXml2::LibXml2 should link with this variable.

[LilyWangL]

I think still LIBXML2_LIBRARIES is not populated correctly.
The missing libraries should first be added to LIBXML2_LIBRARIES variable
and then target LibXml2::LibXml2 should link with this variable.

Due to libiconv and libcharset don't create lib on Linux, so it need not link to them when CMAKE_SYSTEM_NAME STREQUAL "Linux". I think this handle is correct.

[jmelas]

Now it's broken on macOS.
This is my proposition:

if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
    list(APPEND LIBXML2_LIBRARIES m)
else()
    list(APPEND LIBXML2_LIBRARIES ${ICONV_LIBRARIES} ${CHARSET_LIBRARIES} ${LIBLZMA_LIBRARIES} ${ZLIB_LIBRARIES})
    if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
        list(APPEND LIBXML2_LIBRARIES ws2_32)
    endif()
endif()
if(TARGET LibXml2::LibXml2)
    target_link_libraries(LibXml2::LibXml2 INTERFACE ${LIBXML2_LIBRARIES})
endif()

[Neumann-A]

Simply no. This is linking itself. LIBXML2_LIBRARIES is not empty after the find_package call but contains the libxml2 library.

[strega-nil]

Yeah, I think I agree with this; could we switch to the old code, but add target_link_libraries below the second list(APPEND?

[JackBoosY]

Replaced by #14588.