My apologies, I should provide much more detail. In previous setups I had relied on using the Qt installer from qt.io which did a fairly good job of automatically detecting whichever version of Qt that it had installed along side of qtcreator and configuring the IDE to use the latest installed. With the repo version, when opening qtcreator, selecting Tools->Options->Build&Run->Qt Versions, none are auto-detected. However, upon first run a manual entry is already populated which is Qt 4.8.7 from /usr/bin/qmake. Adding another manual entry of /usr/bin/qmake-qt5 and editing the Kits to point to this fixed me up. My laziness got the better of me.
pacman -Qi qtcreator
reports
Name : qtcreator
Version : 3.6.0-2
Description : Lightweight, cross-platform integrated development environment.
Architecture : x86_64
URL : http://qt.digia.com/
Licenses : LGPL
Groups : programming
Provides : None
Depends On : qt5-tools qt5-quick1
Optional Deps : qt5-docs: for the integrated Qt documentation
qt5-webkit-examples: to work with webkit examples [installed]
gdb: for the debugger [installed]
cmake: for cmake project suppport [installed]
git: for git support [installed]
mercurial: for mercurial support [installed]
bzr: for bazaar support [installed]
Required By : None
Optional For : None
Conflicts With : None
Replaces : None
Installed Size : 77.72 MiB
Packager : Anke Boersma <demm@kaosx.us>
Build Date : Fri 08 Jan 2016 11:34:28 PM EST
Install Date : Mon 01 Feb 2016 11:18:09 PM EST
Install Reason : Explicitly installed
Install Script : No
Validated By : SHA256 Sum
