This is a modified copy of the Pawn compiler version 3.2.3664 by Compuphase that fixes some bugs and adds a few features.
See Known compiler bugs for the list of fixed bugs and What's new for the list of features and other notable changes.
The project was originally started as a set of patches aimed to create a compiler that would be compatible with the compiler used in [SA-MP (San Andreas Multiplayer)] (http://sa-mp.com/).
SA-MP uses a modified version of Pawn 3.2.3664 [1] with Windows-only executables, and the developers said that they lost the source code for it which means it can't be ported to other platforms (e.g. Linux) and newly discovered bugs can't be fixed. So the main goal of the project is to re-create changes that were previously made by the devs as well as fix all known compiler bugs.
[1] It's worth noting that the version of the AMX embedded into the SA-MP server seems to be based on an older release of Pawn.