The SYSLIB library is the major component involved in OSHS software porting. It serves as a sink where all the nonportable dirt is gathered and insulated from the rest of the OSHS software world. The library bents the platform in such a way that the OSHS software believes it is running at a genuine OSHS system even when it actually runs on a crappy Windows. The level of tweaking done is really deep; it uses advanced tricks such as Masquerading.
The ancestor of SYSLIB is the "autoconf" program. The main difference between SYSLIB and "autoconf" is that software using "autoconf" uses directly various platform specific calls. The "autoconf" program helps the software to check the platform and choose the right set of calls to be used. But the major problem with "autoconf" is that code which uses it is actually not portable; it is only supporting multiple platforms.
The SYSLIB is a completely different solution. It unifies the interface between the applications and the underlying operating system rather than providing means to check for ways how to do various things and configure the build process accordingly. All the supported platforms are guaranteed to have the same interface which allows the application writers to abandon their portability issues and concentrate to the real work. Also other reasons are behind choosing this way of portability supporting, here is more detailed discussion about this topic.