Update `graphics/zint' to version 2.6.3 (judging by the DISTVERSION, it
looks like it is -RC2, but what really happened is that first, version
2.6.3 was released on Feb 17 2018 which had erroneously listed 2.6.2 as
version in many places. Instead of rerolling the tarball, author have
decided to upload another one, named zint-2.6.3-rc2, on Feb 21 2018).

Add a comment opposite of the DISTVERSION summarizing the above and set
the PORTSCOUT accordingly.

Unfortunately, Zint now requires Qt 5, rather than 4, to build against.