I have made a .pri file named common.pri which has the following code from above:

MOC_DIR = ./build
OBJECTS_DIR = ./build
INCLUDEPATH += usr/lib/x86_64-linux-gnu/
LIBS += -L/usr/lib/x86_64-linux-gnu/ -lfftw3f -lm

and linked it to my other .pro files as include(../common.pri). So, I suppose the linker would scan the fft library before any LIBS in any of my .pro file but still the same error exists.