@fanc999-1 Can you take a look at this? In particular, I don't know what effect LIBXML2_LIB_NO_PKGCONFIG has in pkgconfig. In this PR, since CMake is no longer used for subproject builds I'm not sure this is needed. But maybe in the case where libxml2 was available from CMake and not from the subproject the variable should still be set? But I'm not sure what xml2_dep.type_name() is going to be in that scenario.

I added a commit to set LIBXML2_LIB_NO_PKGCONFIG when not pkgconfig or internal which should be a little closer to the way it was before.

I've merged this PR. Chun-wei Fan can comment afterwards, if he wants to.
But I'm not sure he gets our messages. Usually he replies quickly, but sometimes
he does not get messages sent via GitHub.