Remove unreached line of code from FutureTest
Add -Wunreachable-code compiler option in debug mode

The -Werror option is now checked after the __builtin_unreachable
built-in because the test for __builtin_unreachable would fail with
-Werror.
Merge pull request #77 from magicant/chore/unreachable

Add -Wunreachable-code compiler option in debug mode