[gtkmm-documentation] docs/tutorial/Makefile.am: Try to adapt to Infrastructure/damned-lies

[Kjell Ahlstedt <kjellahl src gnome org>]

commit e2ce4eef4e9a554c88720d1dfbf61552bd2364fb
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Mon Oct 26 17:14:45 2020 +0100

    docs/tutorial/Makefile.am: Try to adapt to Infrastructure/damned-lies
    
    Infrastructure/damned-lies is confused by gtkmm-documentation's
    HELP_FILES = index.docbook, where the specified file is not stored
    in the git repo. Try to make it create a .pot file from the existing
    index-in.docbook.
    
    This is a crazy workaround for an old bug. Even if it works now,
    it may fail after even the slightest modification of
    Infrastructure/damned-lies/stats/utils.py.
    See Infrastructure/damned-lies#58 and gtkmm-documentation#13

 docs/tutorial/Makefile.am | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)
---
diff --git a/docs/tutorial/Makefile.am b/docs/tutorial/Makefile.am
index ad6cb20..eee16e5 100644
--- a/docs/tutorial/Makefile.am
+++ b/docs/tutorial/Makefile.am
@@ -20,8 +20,13 @@ DOCBOOK_STYLESHEET_EPUB ?= http://docbook.sourceforge.net/release/xsl/current/ep
 
 HELP_ID = gtkmm-tutorial
 
-HELP_FILES = \
-       index.docbook
+# This dummy value will perhaps make Infrastructure/damned-lies create a .pot file
+# from index-in.docbook and ignore index.docbook.
+# https://gitlab.gnome.org/Infrastructure/damned-lies/-/issues/58
+HELP_FILES = $(NULL)
+
+# The 'make' command uses the last defined value.
+HELP_FILES = index.docbook
 
 HELP_MEDIA =                                   \
        figures/aspectframe.png                 \