Add a new configuration flag that controls whether the outdated support
for XPointer locations (ranges and points) is enabled.

    --with-xptr-locs          # Autotools
    LIBXML2_WITH_XPTR_LOCS    # CMake

The latest spec for what it essentially an XPath extension seems to be
this working draft from 2002:

    https://www.w3.org/TR/xptr-xpointer/

The xpointer() scheme is listed as "being reviewed" in the XPointer
registry since at least 2006. libxml2 seems to be the only modern
software that tries to implement this spec, but the code has many bugs
and quality issues.

The flag defaults to "off" and support for this extensions has to be
requested explicitly. The relevant API functions are deprecated.