Add a --with-python option

The effect is the same as specifying PYTHON=python3, but a
configure option works better in jhbuild.
https://bugzilla.gnome.org/show_bug.cgi?id=684103

configure.ac

@@ -382,7 +382,16 @@ if test "x$PERL_PATH" = x ; then
 fi
 AC_SUBST(PERL_PATH)
 
-# Need suitable python path for greport
+# option to specify python interpreter to use; this just sets $PYTHON, so that
+# we will fallback to reading $PYTHON if --with-python is not given, and
+# python.m4 will get the expected input
+AC_ARG_WITH(python,
+            AS_HELP_STRING([--with-python=PATH],
+                           [Path to Python interpreter; searches $PATH if only a program name is given; if not given, searches for a few standard names such as "python3" or "python2"]),
+            [PYTHON="$withval"], [])
+if test x"$PYTHON" = xyes; then
+  AC_MSG_ERROR([--with-python option requires a path or program argument])
+fi
 AM_PATH_PYTHON(2.5,,PYTHON="/usr/bin/env python2.5")
 
 

docs/reference/glib/building.xml

@@ -533,6 +533,14 @@
       </para>
     </formalpara>
 
+    <formalpara>
+      <title><systemitem>--with-python</systemitem></title>
+
+      <para>
+        Allows specifying the Python interpreter to use, either as an absolute path,
+        or as a program name. GLib can be built with Python 2 (at least version 2.5)
+        or Python 3.
+      </para>
   </refsect1>
 
 </refentry>