configure tries to check a list of warning flags to use. The check for the -Werror=format-security check however will return false since it checks the flags one-by-one, and format-security only works if used together with -Wformat:

configure:28656: checking if gcc supports flag -Werror=format-security in envvar CFLAGS
configure:28667: gcc -c -Werror -Werror=format-security  -pthread conftest.c >&5
cc1: error: -Wformat-security ignored without -Wformat [-Werror=format-security]
cc1: all warnings being treated as errors
configure:28667: $? = 1

This results in the build never actually using -Werror=format-security (nor -Wformat-security), so these errors will go unchecked.

glib git master, gcc (GCC) 4.7.2 20120921 (Red Hat 4.7.2-2).