[libsigc++2] Add warnings for use with --enable-warnings=fatal

[Murray Cumming <murrayc src gnome org>]

commit 8576b11ca0c4a6cbcc564149d832b22870415b0b
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Nov 12 11:11:39 2015 +0100

    Add warnings for use with --enable-warnings=fatal
    
    Added use -Wsuggest-override and -Wzero-as-null-pointer-constant.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index d48d4f9..ad14296 100644
--- a/configure.ac
+++ b/configure.ac
@@ -62,7 +62,7 @@ AS_IF([test "x$config_error" = xyes],
 # Evaluate the --enable-warnings=level option.
 MM_ARG_ENABLE_WARNINGS([SIGC_WXXFLAGS],
                        [-Wall],
-                       [-pedantic -Wall -Wextra -Wshadow -Wformat-security])
+                       [-pedantic -Wall -Wextra -Wsuggest-override -Wshadow -Wzero-as-null-pointer-constant 
-Wformat-security])
 
 # Offer the ability to omit some API from the library.
 MM_ARG_DISABLE_DEPRECATED_API([SIGCXX])