turn -Werror=null-dereference into a warning

There is unfortunately no way to suppres this witha pragma,
and gcc 9 uncovers quite a few new instaces with enabled LTO that can't/won't be fixed

Related: OS#4123
Change-Id: Ib157bd2e110b271dbe5ac928c98251e016477f56
diff --git a/configure.ac b/configure.ac
index a14c276..b4858c4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -79,7 +79,7 @@
 AX_CHECK_COMPILE_FLAG([-Werror=implicit], [CFLAGS="$CFLAGS -Werror=implicit"])
 AX_CHECK_COMPILE_FLAG([-Werror=maybe-uninitialized], [CFLAGS="$CFLAGS -Werror=maybe-uninitialized"])
 AX_CHECK_COMPILE_FLAG([-Werror=memset-transposed-args], [CFLAGS="$CFLAGS -Werror=memset-transposed-args"])
-AX_CHECK_COMPILE_FLAG([-Werror=null-dereference], [CFLAGS="$CFLAGS -Werror=null-dereference"])
+AX_CHECK_COMPILE_FLAG([-Wnull-dereference], [CFLAGS="$CFLAGS -Wnull-dereference"])
 AX_CHECK_COMPILE_FLAG([-Werror=sizeof-array-argument], [CFLAGS="$CFLAGS -Werror=sizeof-array-argument"])
 AX_CHECK_COMPILE_FLAG([-Werror=sizeof-pointer-memaccess], [CFLAGS="$CFLAGS -Werror=sizeof-pointer-memaccess"])