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>

configs/autoconf.in

@@ -53,7 +53,7 @@ MKDEP_OPTIONS = @MKDEP_OPTIONS@
 INSTALL = @INSTALL@
 
 # Python and flags (generally only needed by the developers)
-PYTHON2 = python
+PYTHON2 = @PYTHON2@
 PYTHON_FLAGS = -t -O -O
 
 # Library names (base name)