Ignore configure backup file created by recent autoreconf too

Vadim Zeitlin requested to merge _VZ_/libxml2:ignore-configure-backup into master

There is currently no way to disable the generation of this annoying file, see https://savannah.gnu.org/support/?110417, so just ignore it.

Just a trivial change to fix a minor annoyance when using libxml2 as a submodule: it appears "dirty" otherwise.

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