Use dependent types to unpack option

Instead of silly thing with default value.
3 jobs for master in 6 minutes and 28 seconds
Name Stage Failure
failed
coq:dev Build

make[2]: *** [src/Congress.vo] Error 1
CoqMakefile:654: recipe for target 'src/Congress.vo' failed
make[1]: *** [all] Error 2
CoqMakefile:319: recipe for target 'all' failed
make[1]: Leaving directory '/builds/jakobbotsch/smart-contracts'
Makefile:7: recipe for target 'all' failed
make: *** [all] Error 2
ERROR: Job failed: exit code 1