Fix build on systems where "python" is python 3.

Fix build on systems where "python" is python 3.

First, it changes autoconf to use a "python2" binary when available,
rather than plain "python" (which is ambiguous).  Secondly, it changes
the Makefiles to use $(PYTHON) $(PYTHON_FLAGS) rather than calling
python directly.

Signed-off-by: Xavier Chantry <chantry.xavier@gmail.com>
Signed-off-by: Matthew William Cox <matt@mattcox.ca>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>