[PATCH 1 of 3] [py3] packaging: allow to run make with python3

[Philippe Pepiot]

# HG changeset patch
# User Philippe Pepiot <philippe.pepiot at logilab.fr>
# Date 1554397608 -7200
#      Thu Apr 04 19:06:48 2019 +0200
# Node ID c9030c811fdff71908344bb17f05bb71d314acc7
# Parent  4ee906aa7b60fb6b113e4dc187fbb5a8f42e557c
[py3] packaging: allow to run make with python3

Use "?=", otherwise the variable cannot be set from environment.

diff --git a/Makefile b/Makefile
--- a/Makefile
+++ b/Makefile
@@ -5,7 +5,7 @@
 # % make PREFIX=/opt/ install
 
 export PREFIX=/usr/local
-PYTHON=python
+PYTHON?=python
 $(eval HGROOT := $(shell pwd))
 HGPYTHONS ?= $(HGROOT)/build/pythons
 PURE=
diff --git a/doc/Makefile b/doc/Makefile
--- a/doc/Makefile
+++ b/doc/Makefile
@@ -6,7 +6,7 @@ GENDOC=gendoc.py ../mercurial/commands.p
 PREFIX=/usr/local
 MANDIR=$(PREFIX)/share/man
 INSTALL=install -c -m 644
-PYTHON=python
+PYTHON?=python
 RSTARGS=
 
 export HGENCODING=UTF-8