Update devel/qtcreator to 4.2.1
This is a pure bugfix release, and takes care of various important bugs:
- define PLIST_SUB so that the pkg-plist does not have to be touched every time
- switch to the tar.xz tarball to save some Mb on disfiles.
Reviewed by: rakuco
Approved by: rakuco (mentor)
Differential Revision: https://reviews.freebsd.org/D9304