[libxml++] Add -Wformat-security to --enable-warnings=fatal.

[Murray Cumming <murrayc src gnome org>]

commit ce298341c1c579d7b179bba5f3ee9ae919bb6e7f
Author: Murray Cumming <murrayc murrayc com>
Date:   Mon Jul 20 11:10:18 2015 +0200

    Add -Wformat-security to --enable-warnings=fatal.
    
    Because we use it in glibmm too. No code changes were necessary.

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index 51e61a5..862d197 100644
--- a/configure.ac
+++ b/configure.ac
@@ -44,7 +44,7 @@ MM_ARG_WITH_TAGFILE_DOC([glibmm-2.4.tag], [glibmm-2.4])
 # Evaluate the --enable-warnings=level option.
 MM_ARG_ENABLE_WARNINGS([LIBXMLXX_WXXFLAGS],
         [-Wall],
-        [-pedantic -Wall -Wextra -Wshadow -Wno-long-long],
+        [-pedantic -Wall -Wextra -Wformat-security -Wshadow -Wno-long-long],
         [G GLIBMM SIGCXX])
 
 # Offer the ability to omit some API from the library.