undefined reference SDL_Init with Qt - c++

Question

I've already gone through questions similar to mine, but none solved my problem.
So I'm trying to use SDL in a Qt Widget (for educational purpose), and I always get and undefined reference on every SDL fonction that I call.
Here is the (minimalist) code I've been using for testing :
#include "mainview.h"
#include <QApplication>
#include "SDL.h"
#undef main
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
MainView w;
w.show();
SDL_Init(SDL_INIT_VIDEO);
SDL_Window* window = SDL_CreateWindowFrom((void*)w.centralWidget->winId());
SDL_Renderer* render = SDL_CreateRenderer(window, -1, SDL_RENDERER_SOFTWARE);
SDL_SetRenderDrawColor(render, 255, 0, 0, 255);
SDL_RenderFillRect(render, NULL);
SDL_RenderPresent(render);
SDL_DestroyWindow(window);
SDL_DestroyRenderer(render);
SDL_Quit();
return a.exec();
}
And here is th .pro file :
#-------------------------------------------------
#
# Project created by QtCreator 2014-10-08T22:37:55
#
#-------------------------------------------------
QT += core gui
greaterThan(QT_MAJOR_VERSION, 4): QT += widgets
TARGET = MapEditor
TEMPLATE = app
SOURCES += main.cpp\
mainview.cpp
HEADERS += mainview.h
FORMS += mainview.ui
INCLUDEPATH += SDL/include/
LIBS += -L SDL/lib/x86/SDL2.lib
According to other answers i've read, it seems the problem comes from the link to the library.
I have never really used this file, or other qmake before, so I am not sure about the path I am using. I work on windows, and according to this article, I should be writing the full path, but then I get access authorization problems (damn Windows).
Furthermore, changing the path to an false one doesn't seems to upset the compiler.
However, when I try to use
LIBS += -L SDL/lib/x86/ -lSDL2.lib
the compiler cannot find the file.
The compiler I'm using is a MinGW 32bits, so I've downloaded the corresponding file on lbsdb.org, and I've tried every library on very sub-folder there was (x86_64, i686,...) but none worked.
And sorry if the way I'm writing is strange, I'm french.

Answer 1

When you have a .lib file, it's easiest to just add the file for linker. You have it subdirectory of your project it seems, but you are probably using a separate build directory (as you should), so the problem is probably that. To fix, use PWD qmake variable:
LIBS += $${PWD}/SDL/lib/x86/SDL2.lib
You might also want to add PWD to include path, because relative path there is relative to directory of the .c file, and you might not always have them all in the same directory.
In a comment you mention something about not being able to use absolute paths... Not sure what you mean by that, but then a way out is to not use shadow build (which is no problem, as long as you remember to do full rebuild when changing build types). If you are using Qt Creator, you can find the project's build settings for each build type under Projects view, and there you have a check box for shadow build.