[libxslt] autotools: Disable parallel Python build

[Nick Wellnhofer <nwellnhof src gnome org>]

commit f47413d13716485a43465efa32ef72cc44ae4a92
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Tue Sep 6 15:13:02 2022 +0200

    autotools: Disable parallel Python build
    
    See https://gitlab.gnome.org/GNOME/libxml2/-/commit/ea8f7c92a9e3f7d89e57b0a1eae0229555da7356

 python/Makefile.am | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)
---
diff --git a/python/Makefile.am b/python/Makefile.am
index bc4449bf..e0c1f5b7 100644
--- a/python/Makefile.am
+++ b/python/Makefile.am
@@ -1,4 +1,8 @@
-# Makefile for libxml2 python library
+# Makefile for libxslt python library
+
+# We use a rule with multiple output files which creates problems with
+# parallel builds.
+.NOTPARALLEL:
 
 SUBDIRS = . tests