[RFA] Add --enable-codesign to gdb's configure

[Tom Tromey]

>> OK for the NEWS part, but I think we should reflect this option in the
>> output of "gdb --configure", because users will want to know whether
>> their GDB was configured that way.

Joel> The new option actually influences the build, not whether GDB provides
Joel> a given feature or not.

I'm not sure there's a way for gdb to know whether it's been signed, I
guess other than invoking codesign itself.  I'm going to check this in,
but if it still needs the change, let me know and I will see if I can
implement it.

Tom