Add -Wformat and -Werror=format-security compiler flags

These are part of the hardening flags used by Debian.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -5067,9 +5067,11 @@ elif enabled ccc; then
 elif enabled gcc; then
     check_optflags -fno-tree-vectorize
     check_cflags -Werror=implicit-function-declaration
+    check_cflags -Werror=format-security
     check_cflags -Werror=missing-prototypes
     check_cflags -Werror=return-type
     check_cflags -Werror=vla
+    check_cflags -Wformat
     enabled extra_warnings || check_disable_warning -Wno-maybe-uninitialized
 elif enabled llvm_gcc; then
     check_cflags -mllvm -stack-alignment=16