configure uses $PYTHON to determine lxml availability, so the make rule
should use the same instead of letting gla11y find a python interpreter
through "env", in case e.g. $PYTHON is python3, and lxml is available in
python3 but not in python (actually python2).

Along the way, rather use PYTHON_FOR_BUILD for coherency.

Change-Id: Ied4c05aca462cc16685c61f36b56bb9e8612d90e
Reviewed-on: https://gerrit.libreoffice.org/50087Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>