For example, if you want to link to a library "/usr/lib/x86_64-linux-gnu/libgstreamer-1.0.so", you should do:
Qt Code: 
  1. LIBS += -lgstreamer-1.0
  2. LIBS += -L/usr/lib/x86_64-linux-gnu/
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So if you are looking for a library named "gthread-2.0" try to search for a file "libgthread-2.0.so" etc...

There are many resources about using shared libraries, for example this one
Try to do some research if you are not sure what those "-L , -I, -l, ..." really mean.