Avoid redefining _FORTIFY_SOURCE

Avoid redefining _FORTIFY_SOURCE

Some compilers have a built-in definition of the _FORTIFY_SOURCE macro
that differs from BIND's preferred setting. This causes errors like
the one quoted below. The solution is to undefine the macro before
defining it. A similar fix was recently committed to glibc.

    <command line>: error: '_FORTIFY_SOURCE' macro redefined
    #define _FORTIFY_SOURCE 2
            ^
    <built-in>: note: previous definition is here
    #define _FORTIFY_SOURCE 0
	    ^

https://sourceware.org/git/glibc.git/commitdiff/35bcb08eaa953c9b