Update CMake's FreeBSD pkg(8) generator

There is an OPTION CPACK for CMake that enables a FreeBSD pkg(8)
generator: after building anything CMake-based, cpack -G FREEBSD
gives you a a ready-to-go installable package.

Well, assuming you turn the option on, and the bugs in the generator
are fixed. Which this patch does. I've tried it with supertux2
and some other bits and pieces and the patch is in a MR upstream.