enable full GCC Stack-smashing protection for all OSes

- change our hardening options to use -fstack-protector-all even for
  Windows builds, as we recently switched to a newer compiler suite
- also removes an obsolete workaround for GCC 4.5
  (https://bugs.launchpad.net/ubuntu/+source/gcc-4.5/+bug/691722), which
  required to first set -fno-stack-protector, before -fstack-protector-all