With the current code, packed structs lead to unaligned memory access,
thus causing SIGBUS on ARM.

Let's disable packing on ARM for now. Since it's only used for mixed
32/64bit jackd installations and we're not facing such a scenario on
ARM, there's little use to worry more atm.

A few internal source files were redeclaring types and macros also
declared in the external jack/types.h and jack/systemdeps.h headers.
Since the external header needs them, delete them from the internal
headers.

As reported by Peter Nelson, waf's check_cfg sets HAVE_OPUS=1 if the
pkg-config check is successful, however, the code requires OPUS custom
headers.

So if a dev package without custom headers is installed, explicitly
reset HAVE_OPUS to 0 to fix the FTBFS.