Pkg-config utility outputs the necessary compiler and linker flags needed to build a given package.
The plug-in will set compiler and linker flags automatically that GCC needs in order to build the selected packages - specifically include paths, libraries and their paths, and some other GCC flags.
Selecting packages happens through Pkg-config property tab where all packages found by pkg-config utility are shown along their descriptions. One can find project specific pkg-config property tab by navigating Properties -> C/C++ Build -> Settings -> Pkg-config.
Pkg-config preference page can be used to change environment variables for PKG_CONFIG_PATH and PKG_CONFIG_LIBDIR. These environment variables are only visible to Eclipse. One shall remember that the pkg-config path cannot contain white spaces (a feature of pkg-config utility). One can find pkg-config preference page by navigating Window -> Preferences -> Pkg-config.