wPOSIX is a minimal POSIX binding whose goal is to help with building cross-platforms applications. This binding is not complete though, as the Win32 API does not provide the necessary support for all POSIX APIs.
To use the wPOSIX binding you need to use a project file, and adding a single with_clause will give you full access to the wPOSIX binding sources and ensure that the proper libraries are passed to the linker.
with "wposix"; project P is for Sources use ...; end P;
To build the application you just need to call gprbuild for the application's project, here p.gpr: