Once the libraries are compiled, the libs and header files are deployed within the build directory, in the sysroot folder.
Library Usage on SDK
When a Linux targeted application is created in the SDK, the wizard menu allows to set your own sysroot folder rather than the default one included in the installation directory. This will add the usr/include path within the sysroot folder, into the compiler include options as well as lib and usr/lib paths into the linker options.
Note: In the release 2017.1 and below there is a well known issue where the linker configuration does not include the --sysroot option.
Above there are some examples of applications using libraries compiled in a petalinux project