[glom] Build: Really require python 3

[Murray Cumming <murrayc src gnome org>]

commit 3b9866dce74a0d7adf0096a316238e7c0fac21ca
Author: Murray Cumming <murrayc murrayc com>
Date:   Tue Dec 4 12:16:40 2018 +0100

    Build: Really require python 3
    
    Note that AM_PATH_PYTHON() sets PYTHON_VERSION,
    so the advice from AX_PYTHON_DEVEL() to set PYTHON_VERSION
    is useless, because that would be overridden.
    So this also corrects the helpful comment in configure.ac,
    to instead advise people to set PYTHON.

 configure.ac | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 60cbf328..1d6e7b84 100644
--- a/configure.ac
+++ b/configure.ac
@@ -234,12 +234,15 @@ AC_CHECK_FUNCS([strptime])
 
 # Get the path to python, to define pyexecdir,
 # so we can use pyexec_LTLIBRARIES in the .am files:
-AM_PATH_PYTHON
+# (This sets PYTHON_VERSION, used by AX_PYTHON_DEVEL later.)
+#
+# To specify a particular python version you must pass PYTHON.
+# For instance, ./configure PYTHON=python3.4 --prefix=/opt/gnome
+AM_PATH_PYTHON([3.0])
 
 # Get the compiler and linker flags for embedding Python.
-# To specify a particular python version you must pass PYTHON_VERSION.
-# For instance, ./configure PYTHON_VERSION="3.4" --prefix=/opt/gnome
-# 
+# This uses the PYTHON_VERSION set by AM_PATH_PYTHON above.
+#
 # See http://www.gnu.org/software/autoconf-archive/ax_python_devel.html
 AX_PYTHON_DEVEL([>= '3.0.0'])