Could not run generate_unaccent_rules.py script when update unicode

Hi, hackers

When I try to update unicode mapping tables using make update-unicode [1],
I encountered an error about following:

generate_unaccent_rules.py --unicode-data-file ../../src/common/unicode/UnicodeData.txt --latin-ascii-file Latin-ASCII.xml >unaccent.rules
/bin/sh: 1: generate_unaccent_rules.py: not found
make: *** [Makefile:33: unaccent.rules] Error 127
make: *** Deleting file 'unaccent.rules'

The generate_unaccent_rules.py is in contrib/unaccent and the Makefile:

# Allow running this even without --with-python
PYTHON ?= python

$(srcdir)/unaccent.rules: generate_unaccent_rules.py ../../src/common/unicode/UnicodeData.txt Latin-ASCII.xml
$(PYTHON) $< --unicode-data-file $(word 2,$^) --latin-ascii-file $(word 3,$^) >$@

It use python to run generate_unaccent_rules.py, However, the ?= operator in
Makefile only check variable is defined or not, but do not check variable is
empty. Since the PYTHON is defined in src/Makefile.global, so here PYTHON
get empty when without --with-ptyhon.

Here are some examples:

japin(at)coltd-devel:~$ cat Makefile
PYTHON =
PYTHON ?= python

test:
echo '$(PYTHON)'
japin(at)coltd-devel:~$ make
echo ''

japin(at)coltd-devel:~$ cat Makefile
PYTHON = python3
PYTHON ?= python

test:
echo '$(PYTHON)'
japin(at)coltd-devel:~$ make
echo 'python3'
python3

japin(at)coltd-devel:~$ cat Makefile
PYTHON =
ifeq ($(PYTHON),)
PYTHON = python
endif

test:
echo '$(PYTHON)'
japin(at)coltd-devel:~$ make
echo 'python'
python
japin(at)coltd-devel:~$ cat Makefile
PYTHON = python3
ifeq ($(PYTHON),)
PYTHON = python
endif

test:
echo '$(PYTHON)'
japin(at)coltd-devel:~$ make
echo 'python3'
python3

Here is a patch to fix this, any thoughts?

diff --git a/contrib/unaccent/Makefile b/contrib/unaccent/Makefile
index 652a3e774c..3ff49ba1e9 100644
--- a/contrib/unaccent/Makefile
+++ b/contrib/unaccent/Makefile
@@ -26,7 +26,9 @@ endif
update-unicode: $(srcdir)/unaccent.rules

# Allow running this even without --with-python
-PYTHON ?= python
+ifeq ($(PYTHON),)
+PYTHON = python
+endif

$(srcdir)/unaccent.rules: generate_unaccent_rules.py ../../src/common/unicode/UnicodeData.txt Latin-ASCII.xml
$(PYTHON) $< --unicode-data-file $(word 2,$^) --latin-ascii-file $(word 3,$^) >$@

[1] https://www.postgresql.org/message-id/MEYP282MB1669AC78EE8374B3DE797A09B6FCA%40MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM

--
Regrads,
Japin Li
ChengDu WenWu Information Technology Co., Ltd.