[gimp-help] configure: fix absent top_srcdir variable.

[Jehan <jehanp src gnome org>]

commit 28d9c2ff875fa696469d01a00029fc9daed171be
Author: Jehan <jehan girinstud io>
Date:   Sun Jun 6 00:10:15 2021 +0200

    configure: fix absent top_srcdir variable.
    
    With double quotes, $(top_srcdir) would look like a shell command to
    run, which is obviously wrong. Instead, switching to single quotes, this
    is not interpreted in the configure script, hence can be replaced by the
    corresponding automake's variable value at the next step.
    
    This fixes the configure error:
    
    > gimp-help/configure: line 2992: top_srcdir: command not found
    
    Note that this error was breaking neither the configure step nor the
    build. It was just displayed during configure.

 configure.ac | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/configure.ac b/configure.ac
index 2ab2fd688..6c51c26a2 100644
--- a/configure.ac
+++ b/configure.ac
@@ -151,7 +151,7 @@ AC_SUBST(DOCBOOK2ODFFLAGS)
 #  Use our customized xml2po
 
 XML2PO='$(top_srcdir)/tools/xml2po.py'
-XML2POFLAGS="--mode=gimphelp --base=$(top_srcdir)/"
+XML2POFLAGS='--mode=gimphelp --base=$(top_srcdir)/'
 AC_SUBST(XML2PO)
 AC_SUBST(XML2POFLAGS)