Qt Creator: Release build won't compile - c++

Question

I've written a relatively simple Qt program that I'm trying to compile for windows with MinGW under Qt Creator. The Debug version builds and runs without problem, but when I try to build the Release version I get these errors
main.cpp:-1: error: undefined reference to `_imp___ZN12QApplicationC1ERiPPci'
main.cpp:-1: error: undefined reference to `_imp___ZN7QWidget4showEv'
main.cpp:-1: error: undefined reference to `_imp___ZN12QApplication4execEv'
main.cpp:-1: error: undefined reference to `_imp___ZN12QApplicationD1Ev'
main.cpp:-1: error: undefined reference to `_imp___ZN12QApplicationD1Ev'
:-1: error: release/main.o: bad reloc address 0x13 in section `.eh_frame'
collect2.exe:-1: error: error: ld returned 1 exit status
The errors seem to suggest that one or more of the core Qt libraries cannot be found, however, the .pro file contains the line QT += core gui widgets and there is nothing Debug/Release specific. I've tried cleaning/rebuilding the project and restarting Qt Creator, to no avail. I have no problems compiling Release builds for other projects. I'm using Qt version 5.2.1. Any ideas?
Update:
I've singled out this line in the .pro file as the cause
win32: LIBS += -L"C:/Program Files (x86)/MATLAB/R2015b/bin/win32" -llibmat -llibmx
Inside the makefile I see this
LIBS = -lglu32 -lopengl32 -lgdi32 -luser32 -lmingw32 -lqtmain "-LC:/Program Files (x86)/MATLAB/R2015b/bin/win32" -llibmat -llibmx -LC:\Qt\Qt5.2.1\5.2.1\mingw48_32\lib -lQt5Widgets -lQt5Gui -lQt5Core
I suspect the problem lies in the order in which the link libraries are listed in the makefile. I vaguely recall encountering a similar problem in the past. How can I force qmake to place the Matlab libraries at the end of the list?