[jhbuild/configure-prefer-py3] autotools: allow python3 and prefer it if both are available

[Christoph Reiter <creiter src gnome org>]

commit b079d4ffe57a0beec09a86b2c022b0dea9be1942
Author: Christoph Reiter <reiter christoph gmail com>
Date:   Mon Sep 23 19:33:32 2019 +0200

    autotools: allow python3 and prefer it if both are available
    
    In case --with-python is passed we allow 2.7+ and 3.5+.
    In case nothing is passed we try to find 3.5+ first and then
    fall back to 2.7+

 .gitlab-ci.yml |  2 +-
 configure.ac   | 30 ++++++++++++++++++++++++------
 2 files changed, 25 insertions(+), 7 deletions(-)
---
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 507066dc..b88305ae 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -36,7 +36,7 @@ test-py3:
 ub19.04-py2-glib:
   image: registry.gitlab.gnome.org/gnome/jhbuild/jhbuild:v3
   script:
-    - ./autogen.sh
+    - ./autogen.sh  --with-python=python2
     - make
     - make install
     - export PATH=$HOME/.local/bin:$PATH
diff --git a/configure.ac b/configure.ac
index 9d30ecc8..4953ee50 100644
--- a/configure.ac
+++ b/configure.ac
@@ -26,14 +26,32 @@ if test -n "$PYTHON" && ! command -v "$PYTHON" > /dev/null; then
        AC_MSG_ERROR([Python interpreter $PYTHON does not exist])
 fi
 
-# If PYTHON is set we use it, otherwise we only allow Python 2 until the Python 3 port is complete
+# All this is supposed to do is check the Python version against python_min_ver
+# and python3_min_ver and prefer Python 3.x if nothing is set.
+m4_define(python_min_ver, 2.7)
+m4_define(python3_min_ver, 3.5)
 if test -n "$PYTHON"; then
-       AM_PATH_PYTHON([2.7])
+       AM_PATH_PYTHON([python_min_ver])
+       AM_PYTHON_CHECK_VERSION([$PYTHON], [3.0],
+                               build_py3k=true,
+                               build_py3k=false)
+       if test $build_py3k = true ; then
+               AC_MSG_CHECKING([for $PYTHON >=] python3_min_ver)
+               AM_PYTHON_CHECK_VERSION([$PYTHON], python3_min_ver,
+                                       [AC_MSG_RESULT(yes)],
+                                       [AC_MSG_ERROR(too old)])
+       fi
 else
-       AM_PATH_PYTHON([], [
-               AS_CASE(["$PYTHON_VERSION"],
-                       [2.*], [],
-                       [AC_MSG_ERROR([JHBuild only works with python 2])])])
+       AM_PATH_PYTHON([python3_min_ver], [], [
+               unset PYTHON;
+               # XXX: AM_PATH_PYTHON caching is broken and always returns the result of the first use
+               unset am_cv_pathless_PYTHON;
+               AM_PATH_PYTHON([python_min_ver], [
+                       AS_CASE(["$PYTHON_VERSION"],
+                               [2.*], [],
+                               [AC_MSG_ERROR([JHBuild requires Python ] python_min_ver [or] 
python3_min_ver)])
+               ])
+       ])
 fi
 
 PKG_PROG_PKG_CONFIG