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

[Murray Cumming <murrayc src gnome org>]

commit a32ae77d32b219d975deeba818d853e23882d2c1
Author: Murray Cumming <murrayc murrayc com>
Date:   Thu Nov 19 11:58:12 2015 +0100

    Revert "Use -Wzero-as-null-pointer-constant with --enable-warnings=fatal"
    
    This reverts commit 1755af6e6dc62157737c18e3ed8f5beb4b06e25a.
    Because it breaks the build do the (correct in C) use of NULL for null
    pointers in the glib headers.
    See https://bugzilla.gnome.org/show_bug.cgi?id=757979#c1

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