Do not use fixed python executable name and use AM_PROG_PYTHON

On many systems now python executable it is python3.
On my system it the case and by this muild failed with:

make[2]: Leaving directory '/home/tkloczko/rpmbuild/BUILD/backends-1.0.31/backend'
make[2]: Entering directory '/home/tkloczko/rpmbuild/BUILD/backends-1.0.31/backend'
Generating pixma/pixma_sane_options.h from pixma/pixma.c
/bin/sh: python: command not found
make[2]: *** [Makefile:7317: pixma/pixma_sane_options.h] Error 127
make[2]: Leaving directory '/home/tkloczko/rpmbuild/BUILD/backends-1.0.31/backend'
make[2]: *** Waiting for unfinished jobs....

Using AM_PROG_PYTHON() nmacro cllows avoide tha by hecks current version of the
python and allw to use custom python executable by execute configure by:

$ PYTHON=</some/python> ./configure <params>

AM_PROG_PYTHON aclocal macro propagates to automake files $(PYTHON) variable
which could be used on geberate instaleable scripts on duting build process like
it is now in backend/Makefile.am.

Currently minum version of required vesion of the python is python >= 3.

backend/Makefile.am

@@ -947,10 +947,10 @@ $(srcdir)/pixma/pixma.c: \
 
 $(srcdir)/pixma/pixma_sane_options.h:
 	@echo Generating $@ from $(@D)/pixma.c
-	@(cd $(@D); python scripts/pixma_gen_options.py h < pixma.c > $(@F))
+	@(cd $(@D); $(PYTHON) scripts/pixma_gen_options.py h < pixma.c > $(@F))
 $(srcdir)/pixma/pixma_sane_options.c:
 	@echo Generating $@ from $(@D)/pixma.c
-	@(cd $(@D); python scripts/pixma_gen_options.py   < pixma.c > $(@F))
+	@(cd $(@D); $(PYTHON) scripts/pixma_gen_options.py   < pixma.c > $(@F))
 
 EXTRA_DIST += pixma/pixma_sane_options.c
 EXTRA_DIST += pixma/pixma_sane_options.h

configure.ac

@@ -36,6 +36,7 @@ dnl ******************************************************************
 AC_PROG_CC
 AC_PROG_CXX
 AM_PROG_CC_C_O
+AM_PATH_PYTHON(3)
 sane_save_CC=$CC
 AC_PROG_CC_C99                  dnl enables extensions to ISO C99 :-(
 AS_IF([test xno != "x$ac_cv_prog_cc_c99"],