Fix _FORTIFY_SOURCE redefinition error

Fix _FORTIFY_SOURCE redefinition error

Some systems define _FORTIFY_SOURCE by default. It conflicts with our use
of that macro when --enable-extraprotection is passed to the configuration
script.

The solution is to undefine it before redefining it with value 2.

If a higher _FORTIFY_LEVEL level ever appears, it will have to be used in
this program to benefit from the highest protection possible.