[gtkmm] configure.ac: Copy COPYING from the source directory

[Kjell Ahlstedt <kjellahl src gnome org>]

commit 91b366cd8bd09dacc35f40aaf2e308ea51e09059
Author: Kjell Ahlstedt <kjellahlstedt gmail com>
Date:   Wed Mar 20 15:53:18 2019 +0100

    configure.ac: Copy COPYING from the source directory

 configure.ac | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/configure.ac b/configure.ac
index d286b8ab..856f9a99 100644
--- a/configure.ac
+++ b/configure.ac
@@ -168,5 +168,5 @@ AC_CONFIG_COMMANDS([MSVC_NMake/gdkmm/gdkmmconfig.h],
 AC_CONFIG_COMMANDS([MSVC_NMake/gtkmm/gtkmmconfig.h],
                    [cp -f gtk/gtkmmconfig.h MSVC_NMake/gtkmm/gtkmmconfig.h])
 AC_CONFIG_COMMANDS([win32_installer/lgpl.txt],
-                   [cp -f COPYING win32_installer/lgpl.txt])
+                   [cp -f "$srcdir/COPYING" win32_installer/lgpl.txt])
 AC_OUTPUT