[libsigc++2] Use -Wformat-security with --enable-warnings=fatal.

[Murray Cumming <murrayc src gnome org>]

commit 264cfc06dbb26a3471900a3425bec336f4028405
Author: Murray Cumming <murrayc murrayc com>
Date:   Sat Jul 18 11:21:45 2015 +0200

    Use -Wformat-security with --enable-warnings=fatal.
    
    Because we use it in glibmm already. No code changes were necessary.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index caf79fd..91158be 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])
+                       [-pedantic -Wall -Wextra -Wshadow -Wformat-security])
 
 # Offer the ability to omit some API from the library.
 MM_ARG_DISABLE_DEPRECATED_API([SIGCXX])