Fix build with bleeding edge GLib

CTags defines __unused__ and __printf__, which not only are reserved
identifiers, but actually are used by GNUC as arguments of the
__attribute__() extension.  This used to work because no code seeing
those definitions was trying to use them as __attribute__() argument,
but a recent change in GLib made it use it in atomic operation, which
are used by the tagmanager, which itself includes the CTags header
defining those, leading to a weird build failure -- since __unused__
expanded to an unexpected value.

To fix this, rename CTag's __ununsed__ to UNUSED and __printf__ to
PRINTF.