configure: Make sure Python is available for static builds

We need Python to create files that reference the plugin constructors.
Without it, empty files are created and plugins can't be loaded.

Fixes #3349.

configure.ac

@@ -1379,6 +1379,9 @@ AC_CACHE_CHECK(
 )
 if test "x$ss_cv_static_plugin_constructors" = xyes; then
 	static_plugin_constructors=true
+	if test "$PYTHON" = ":"; then
+		AC_MSG_FAILURE([Python is required to resolve plugin constructors statically])
+	fi
 fi
 
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