[glibmm] Use -Wzero-as-null-pointer-constant with --enable-warnings=fatal

[Murray Cumming <murrayc src gnome org>]

commit 1755af6e6dc62157737c18e3ed8f5beb4b06e25a
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Nov 12 11:19:02 2015 +0100

    Use -Wzero-as-null-pointer-constant with --enable-warnings=fatal
    
    But see mm-common bug #757979 .

 configure.ac |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/configure.ac b/configure.ac
index e8dd8ce..fadb65b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -116,7 +116,7 @@ GLIBMM_ARG_ENABLE_DEBUG_REFCOUNTING
 # Evaluate the --enable-warnings=level option.
 MM_ARG_ENABLE_WARNINGS([GLIBMM_WXXFLAGS],
                        [-Wall],
-                       [-pedantic -Wall -Wextra -Wformat-security -Wsuggest-override -Wshadow 
-Wno-long-long],
+                       [-pedantic -Wall -Wextra -Wformat-security -Wsuggest-override -Wshadow 
-Wzero-as-null-pointer-constant -Wno-long-long],
                        [G SIGCXX])
 
 # Offer the ability to omit some API from the library,