ripped from Debian

# DP: Turn on -Wformat -Wformat-security by  default for C, C++, ObjC, ObjC++.

--- gcc/c-common.c
+++ gcc/c-common.c
@@ -277,7 +277,7 @@
 /* Warn about format/argument anomalies in calls to formatted I/O functions
    (*printf, *scanf, strftime, strfmon, etc.).  */
 
-int warn_format;
+int warn_format = 1;
 
 /* Warn about using __null (as NULL in C++) as sentinel.  For code compiled
    with GCC this doesn't matter as __null is guaranteed to have the right
--- gcc/c.opt
+++ gcc/c.opt
@@ -228,7 +228,7 @@
 Warn about format strings that contain NUL bytes
 
 Wformat-security
-C ObjC C++ ObjC++ Var(warn_format_security) Warning
+C ObjC C++ ObjC++ Var(warn_format_security) Init(1) Warning
 Warn about possible security problems with format functions
 
 Wformat-y2k
--- gcc/doc/invoke.texi
+++ gcc/doc/invoke.texi
@@ -2802,6 +2802,9 @@
 @option{-Wformat-nonliteral}, @option{-Wformat-security}, and
 @option{-Wformat=2} are available, but are not included in @option{-Wall}.
 
+NOTE: In Gentoo, this option is enabled by default for C, C++, ObjC, ObjC++.
+To disable, use @option{-Wformat=0}.
+
 @item -Wformat-y2k
 @opindex Wformat-y2k
 @opindex Wno-format-y2k
@@ -2849,6 +2852,11 @@
 in future warnings may be added to @option{-Wformat-security} that are not
 included in @option{-Wformat-nonliteral}.)
 
+NOTE: In Gentoo, this option is enabled by default for C, C++, ObjC, ObjC++.
+To disable, use @option{-Wno-format-security}, or disable all format warnings
+with @option{-Wformat=0}.  To make format security warnings fatal, specify
+@option{-Werror=format-security}.
+
 @item -Wformat=2
 @opindex Wformat=2
 @opindex Wno-format=2