FFMPEG and QT Creator

Hi, guys! I'm trying to use FFMPEG libraries on Qt Creator. I've followed the procedure to install the libraries on my Ubuntu (http://ffmpeg.org/trac/ffmpeg/wiki/UbuntuCompilationGuideLucid). Now, what I want to do, is to include these libraries on my own project with Qt Creator. I've tried to modify the .pro file as the following:

INCLUDEPATH += /usr/local/include/libavcodec \
                /usr/local/include/libavdevice \
                /usr/local/include/libavutil \
                /usr/local/include/libavfilter \
                /usr/local/include/libavformat \
                /usr/local/include/libswscale \
                /usr/local/include/libpostproc \
                /usr/local/include/libswresample \



LIBS += /usr/local/lib/libavcodec.a \
/usr/local/lib/libavdevice.a \
/usr/local/lib/libavfilter.a \
/usr/local/lib/libavformat.a \
/usr/local/lib/libavutil.a \
/usr/local/lib/libmp3lame.a \
/usr/local/lib/libpostproc.a \
/usr/local/lib/libswresample.a \
/usr/local/lib/libswscale.a \
/usr/local/lib/libvpx.a \
/usr/local/lib/libx264.a \
/usr/local/lib/libyasm.a \

I've tried a lot of way to set the above parameters, but it is stilling give me the same error. The error is:
/home/doctor/ffmpegprova/main.cpp:31: undefined reference to `avcodec_register_all()'

This error comes up when I try to call one function inside the libraries, when I comment all the functions inside the libraries, then the error goes away.
The code that I've tried is the following:
#include <libavutil/opt.h>
#include <libavcodec/avcodec.h>
#include <libavutil/channel_layout.h>
#include <libavutil/common.h>
#include <libavutil/imgutils.h>
#include <libavutil/mathematics.h>
#include <libavutil/samplefmt.h>

#include <QtGui/QApplication>
#include "mainwindow.h"

int main(int argc, char *argv[])
{
    /* register all the codecs */
     avcodec_register_all(); //<---this function gives the error

    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}


What I have to write inside .pro file? Thank you in advance.