[gtkmm] Use -Wsuggest-override with --enable-warnings=fatal.

[Murray Cumming <murrayc src gnome org>]

commit 046de86168d01bd5b35ccf755af9d87e2cc2b227
Author: Murray Cumming <murrayc murrayc com>
Date:   Tue Oct 27 09:43:46 2015 +0100

    Use -Wsuggest-override with --enable-warnings=fatal.
    
    This warning is available in g++ 5 (or at least 5.2).

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 2f02a58..7e16f46 100644
--- a/configure.ac
+++ b/configure.ac
@@ -139,7 +139,7 @@ AC_CHECK_FUNCS([mkfifo])
 # Evaluate the --enable-warnings=level option.
 MM_ARG_ENABLE_WARNINGS([GTKMM_WXXFLAGS],
                        [-Wall],
-                       [-pedantic -Wall -Wextra -Wformat-security -Wshadow -Wno-long-long -DGSEAL_ENABLE],
+                       [-pedantic -Wall -Wextra -Wformat-security -Wshadow -Wsuggest-override -Wno-long-long 
-DGSEAL_ENABLE],
                        [G PANGO ATK GDK GDK_PIXBUF GTK GLIBMM ATKMM PANGOMM CAIROMM SIGCXX])
 
 AC_CONFIG_FILES([Makefile