Improve ease of debugging of client/server examples

- Define new CFLAGS in debug mode:
  - enable full embedding of debugging symbols
  - lower optimisation to avoid 'jumping' in gdb
  - enable stricter compiler warnings
- mitigate compiler warnings in client example
- Build client and server executables as binaries to
  allow them to be invoked directly by gdb

This reverts commit 544ee97e.

Travis osx

This is probably perl which comes with precise is too old.  Upgrading
travis build to use trusty fixes the issue.

Travis: Remove boringssl build