New port: devel/gwenhywfar-qt5, the Gwenhywfar Qt5 support library

While here:

• Clean up if statements in the master port
• Substitute the major.minor version in all gwenhywfar pkg-plists

devel/gwenhywfar/files/patch-configure had to be reworked, but the
changes only affect the new port and the other changes do not
affect the packages, so no PORTREVISON bump.