[gimp] devel-docs: fix automake when --disable-gtk-docs is used

[Jehan <jehanp src gnome org>]

commit 64c786e19ab4d09a49ae2df3c7b5b7a823dfd030
Author: Andrzej Hunt <andrzej ahunt org>
Date:   Tue Aug 10 17:15:23 2021 +0200

    devel-docs: fix automake when --disable-gtk-docs is used
    
    A number of autmake files under devel-docs expand "CLEANFILES += ...". This
    depends on gtkdocize setting up gtk-docs.make with (amongst other things)
    "CLEANFILES = ...". However when running autogen.sh with --disable-gtk-docs,
    gtk-docs.make is only populated with EXTRA_DIST - meaning CLEANFILES is not
    set, and automake fails to parse the devel-docs automake files.
    
    Therefore we need to explicitly set CLEANFILES when --disable-gtk-docs is
    used to avoid seeing errors such as the following when running autogen:
      devel-docs/libgimp/Makefile.am:85: error: CLEANFILES must be set with '=' before using '+='
    
    The CLEANFILES extension was first added in:
      0052803313 (devel-docs: REVERT THIS once module.actions get removed automatically, 2020-10-25)

 autogen.sh | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/autogen.sh b/autogen.sh
index 13b819f46d..eb66aefb59 100755
--- a/autogen.sh
+++ b/autogen.sh
@@ -282,6 +282,7 @@ $LIBTOOLIZE --force || exit $?
 if test x$enable_gtk_doc = xno; then
     if test -f gtk-doc.make; then :; else
        echo "EXTRA_DIST = missing-gtk-doc" > gtk-doc.make
+       echo "CLEANFILES = " >> gtk-doc.make
     fi
     echo "WARNING: You have disabled gtk-doc."
     echo "         As a result, you will not be able to generate the API"