pkg-config supports `Requires.private` and `Libs.private` fields for
static linking. However, if you're building a dynamic binary, then
pkg-config will use the non-private fields, even if just the static
libxml2 is available. This will result in libxml2 being underlinked,
causing the build to fail. The solution is to fold the private fields
into the non-private fields when the shared libxml2 is not being built.

This works for Autotools and CMake. Meson also knows how to handle this
when it automatically generates pkg-config files.