[libxml2] Improve pc file

[dg0yt]

• What does your PR fix?

  The pc file did not list any extra libraries needed for static linking. This PR adds them, e.g. for x64-linux-dbg:
  Libs.private: -pthread -lm -L/vcpkg/installed/x64-linux/debug/lib/pkgconfig/../../lib -llzmad -pthread -lz
  This helps with fixes the libxml2 test in gdal configure.

  In addition, the PR also shortcuts a copying step introduced in my previous PR.

• Which triplets are supported/not supported? Have you updated the CI baseline?

  all, no.

• Does your PR follow the maintainer guide?

  yes

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  yes

• Remarks

  • Similar configuration is done in upstream HEAD's CMakeLists.txt. CC @JackBoosY
  • Some requirements are collected via x_vcpkg_pkgconfig_get_modules, in order to easily to resolve recursively and with debug postfixes as needed. CC @Neumann-A

[dg0yt]

Probably there is also a dependency on CMAKE_DL_LIBS, given that HAVE_DLOPEN is defined for linux and osx. Not sure if this desired for static builds at all.

[dg0yt]

x64_windows_static_md: paraview: FAILED: Qt/Components/pqComponents_autogen/timestamp - seems unrelated.

[Neumann-A]

x64_windows_static_md: paraview: FAILED: Qt/Components/pqComponents_autogen/timestamp - seems unrelated.

@dg0yt: Fixed by #17706. Is a CMake autogen regression

[dg0yt]

Thanks for reviewing.

After submitting the PR, I started wondering whether this usage of x_vcpkg_pkgconfig_get_modules isn't a anti-pattern:
The result is only used to insert Libs into the pc file. This result can probably also be achieved just by adding corresponding Requires: (or Requires.private: ?) modules to the the pc file template. This would avoid the expensive extra acquisition and run of pkg-config when building the port.

[dg0yt]

This result can probably also be achieved just by adding corresponding Requires: (or Requires.private: ?) modules to the the pc file template.

Confirmed. I will update the PR unless someone objects.

[dg0yt]

adding corresponding Requires: (or Requires.private: ?) modules to the the pc file template.

Done: Requires.private.

[dg0yt]

#17945 will also solve the pc file problem. If that PR is ready for merge soon, this PR becomes obsolete.

[JackBoosY]

I prefer to merge this PR first because my PR needs to update cmake to the latest.