improve architecture detection

Recognize ppc64, ppc, s390x, s390 CPUs, default to 'none' instead of
i686 (which tries to build with SSE and fails on non-x86 architectures).
don't overwrite necessary compiler flags

Distinguish flags influencing optimization (overridable) from those
necessary for building (e.g. for using SSE).
ignore "unofficial" states in jack_sync_callback()
Merge branch 'master--ignore-JackTransportNetStarting' of https://github.com/nphilipp/ardour