If your library provides the pkgconfig.pc file, you can use link_pkgconfig in your .pro file as:
CONFIG += link_pkgconfig PKGCONFIG += quazip
If the library provides a command-line utility for obtaining compiler flags (for example, postgresql), then you can call it and assign the output to be added to the appropriate variable
INCLUDEPATH += $$system(pg_config --includedir)
If the library itself was also written in Qt, it is unlikely, but it is possible that it provides a Qt module, for example qwt. In this case, you can simply add to your .pro file:
CONFIG += qwt
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