When you use -m68030 -msoft-float, your own code will not use the FPU, but it will be linked with the -m68020-60 MiNTLib. This is why your executable fails to run if you don't have an FPU.
I only add that in case you do want to use 68000 version anyway, you can, there's nothing wrong anymore with mixing 68000 libc.a with a 68030 (either with hard or soft float) app. But of course you have to specify it manually, using -L/-l switches + disable automatic libc linking.