Hello I am new here and I have a problem with compiling my code, I get the following error:

/home/maxfkira/Demo/lib/libdemo.a(TracHold.o): In function `exitTrac':
TracHold.c: (.text+0x1b): undefined reference to `fftwf_malloc'
TracHold.c: (.text+0x37): undefined reference to `fftwf_malloc'
TracHold.c: (.text+0x53): undefined reference to `fftwf_free'
TracHold.c: (.text+0x6f): undefined reference to `fftwf_free'

I am using a .pro file and I have installed fftw-3.2.1. The linker's library search path is /usr/lib/x86_64-linux-gnu and the files are also present there.

TEMPLATE = app
TARGET = TRACconsole
DESTDIR = ../bin
QT += core \
gui
SOURCES += main.cpp
LIBS += -L/home/maxfkira/Demo/lib -ldemo
POST_TARGETDEPS += ../lib/libdemo.a
MOC_DIR = ./build
OBJECTS_DIR = ./build
INCLUDEPATH += /usr/lib/x86_64-linux-gnu
LIBS += -L/usr/lib/x86_64-linux-gnu -lfftw3 -lfftw3f -lm

I am unable to identify where the problem is. I would be really grateful for some help.
Thank you.