I get it though. hip developers don't like to support distributions that are actually stable.
Really sucks the current software dev culture, they want us all to use distributions that are fresh and full of quirks that haven't been ironed out.
Qt 5.5 is stable and it is the recommended version of Qt to use. Qt 4 is legacy and is being deprecated. Support for Qt 4 is still there in case people still want to use it, but it is still outdated software. Qt doesn't recommend that Qt 4 be used.
That's not what ./configure --help says :
--with-gui[=no|qt4|qt5|auto]
build bitcoin-qt GUI (default=auto, qt5 tried first)
That implies that auto is the default, not qt5 - and that the default will try qt4 if it doesn't find qt5
So either it is a bug in detection or it is a bug in the --help output of ./configure
Apparently it is a bug. See
https://github.com/bitcoin/bitcoin/issues/7189