NEWS: Zsh completion support isn't new in 6.0

There have been several fixes to the Zsh completion during the current
development cycle, but the completion was initially implemented
already in 4.0.
build-sys: Always define bluetooth related HAVE_* variables

I don't know if it can cause any problems if HAVE_BLUEZ_4,
HAVE_BLUEZ_5, HAVE_BLUEZ, HAVE_BLUEZ_5_OFONO_HEADSET or
HAVE_BLUEZ_5_NATIVE_HEADSET are undefined when the corresponding
features are not enabled, but it certainly won't hurt to define the
variables also when the features are not enabled.
build-sys: Fix the BlueZ 5 native headset backend check

If the libbluetooth headers aren't available, we shouldn't treat that
as an error unless --enable-bluez5-native-headset has been explicitly
given to configure.

BugLink: https://bugs.freedesktop.org/show_bug.cgi?id=86582