Add -Wformat-security to build

[sjkelly]

This enhances some printing security:

At present, this warns about calls to printf and scanf functions where
the format string is not a string literal and there are no format arguments,
as in printf (foo);. This may be a security hole if the format string came
from untrusted input and contains ‘%n’. (This is currently a subset of what
-Wformat-nonliteral warns about, but in future warnings may be added to
-Wformat-security that are not included in -Wformat-nonliteral.)

[fingolfin]

Looks good to me