travis/coverity: hotfix, alwasy return success

It seems the coverity script likes to return 1 (error) when the current
branch does not follow its pattern. Temporarily disable that.
travis/coverity: hotfix #2, unbreak build

I hadn't spotted that the `sh -e` commandline was being used. I *think* this
is what's causing the exit 0 line not to run. Pray for success.

It's a real shame I can't this locally, what a mess.
Make it easier to turn the bundled dependencies off.
Fix unused variable when NDEBUG is defined.

Occurs when compiling with:
  rm -rf build/ && make clean && make cmake CFLAGS='-DNDEBUG' && make
  ^--important
Merge #714 'Fix unused variable when NDEBUG is defined'