I figured it out, and will post this up in case anyone else runs into this issue:
I was trying to run Qt5 on Raspbian jessie on a Pi 3 model B.
Qt5 and qt Creator were installed successfully, but when I created a simple program and tried to run it I encountered an error message stating that an error was encountered in qmake.
Running qmake from the command line provided the additional information regarding the stash file, which I previously posted.
This error was caused by the Qt IDE not having the settings configured to use the GCC compiler.
There is a walkthough on setting up the IDE for GCC on this page:
http://helloraspberrypi.blogspot.com/20 ... ry-pi.html
just head the the part of the video after sudo apt-get install qt5-default
and sudo apt-get install qtcreator
I do still have the problem where the compiler settings are wiped out every time Qt is shut down, but there is a solution in the comments of that blog which I will try out.