The default compiler in DragonFly is going to change over from GCC 4.7 to GCC 5.x very soon, to match the GCC 5.1 release. This means that packages built for DragonFly-master won’t be compatible with the old ones. You will need to reinstall packages when you next ‘pkg install’. John Marino has an extensive writeup detailing what’s needed, and the actual change is some days off.
If you are using DragonFly 4.0.x (the release), this doesn’t affect you at all.