Log Message:
py-libxml2: Add explanation of incorrect header file processing

It remains to turn the explanation of what is going wrong into a fix,
awaiting comments and perhaps the branch.

The esssence of the problem is that at least on older macOS, there is
a native libxml2 with legacy enabled, not intended to be used by the
package, and a native iconv, which is intended to be used, and the
ordering of -I results in the native libxml2 headers being used.