#7 64-bit system compatibility

[Mart Raudsepp]

In Gentoo Linux we have reports that wxchecksums-1.2.0
doesn't work out of the box on 64-bit systems, such as
amd64 in a 64-bit userland.
http://bugs.gentoo.org/show_bug.cgi?id=113411 has the
details.
Attaching patch that is now applied in Gentoo, but the
open questions about alignment probably should be
addressed about the #ifdef __LP64__ case (should it be
different macro for 64-bit or not?).

[Mart Raudsepp]

64-bit compatibility patch