`make` is `gmake` on FreeBSD, so add a variable to include this change in command.

The Rust code and some built-in compiler functions (__divdc3/__divsc3/__divxc3)
complained about missing floating-point stuff:
- fmax, fmaxf, fmaxl, logb, logbf, logbl, scalbnl, trunc, floor, ceil.

Passing `--gc-sections` to the linker seems to fix this.