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.