@life0fpi
I'm not sure if you figured this out last month, but I installed Qt Creator and played around with this today.
I right-clicked -> Add Library... -> External Library -> filled out the dialog with reasonable inputs for a LeapSDK in my project directory. It created the following entries in my *.pro file.
win32: LIBS += -L$$PWD/LeapSDK/lib/x86 -lLeap
INCLUDEPATH += $$PWD/LeapSDK/include
DEPENDPATH += $$PWD/LeapSDK/include
At times I forgot to go to Build -> Run qmake, but once that was done it compiled then reported linker errors just as you're seeing.
Upon looking at the Kit, it was set to Desktop Qt 5.4.1 MSVC2013 64bit, which certainly wasn't what I had expected. By switching to the x64 DLL I was then able to link and run. Alternatively, after setting it back to x86 then using the MSVC2010 OpenGL 32bit kit (as I happen to have Visual Studio 2010 installed too), it worked.
To switch your project's kit, see the following screenshot.
Oddly, when I installed Qt Creator it did not come with any 32-bit kit for MSVC 2013. It only contains msvc2010_opengl, msvc2012_opengl, and msvc2013_64. Perhaps the same thing happened to you.