Fix CMake configure failing on BSD

[hellium6]

This PR allows to successfully build on NetBSD 10. Related to #45 .

FREEBSD generator seems to work on NetBSD and other BSDs:

The CPack FreeBSD generator may be used to create pkg(8) packages -- these may be used on FreeBSD, DragonflyBSD, NetBSD, OpenBSD, but also on Linux or OSX, depending on the installed package-management tools -- using CPack.

https://cmake.org/cmake/help/latest/cpack_gen/freebsd.html

So adding this should get rid of the error mentioned in the issue in those BSDs.

"It tries to re-use packaging information that may already be specified for Debian packages for the CPack DEB Generator." CMakeLists.txt already has CPACK_DEBIAN_* values, so there should be no need to specify them for FreeBSD.

NetBSD has a different prefix for packages installed with pkgsrc or pkgin:

$ uname -smr
NetBSD 10.0 amd64
$ pkgconf --libs libsodium
-Wl,-R/usr/pkg/lib -L/usr/pkg/lib -lsodium

It causes linking to fail:

...
[100%] Linking CXX executable spicypass
ld: cannot find -lsodium
ld: cannot find -lgtk-3
ld: cannot find -lgdk-3
ld: cannot find -lpangocairo-1.0
ld: cannot find -lpango-1.0
ld: cannot find -lharfbuzz
ld: cannot find -latk-1.0
ld: cannot find -lcairo-gobject
ld: cannot find -lcairo
ld: cannot find -lgdk_pixbuf-2.0
ld: cannot find -lgio-2.0
ld: cannot find -lgobject-2.0
ld: cannot find -lglib-2.0
gmake[2]: *** [CMakeFiles/spicypass.dir/build.make:196: spicypass] Error 1
...

So had to add CMAKE_CXX_FLAGS for NetBSD