Each library will usually have a ".pri" files (Qt project include) that adds the necessary elements to each variable.

The application then simply includes those and thus gets the full modification.

E.g. in your case, which apparently has headers and library in the source directory

Qt Code: 
  1. INCLUDEPATH += $$PWD
  2. DEPENDPATH += $$PWD
  3. LIBS += -l$$PWD -lnameoflib
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Cheers,
_