splfr: C compiler and related tools from Microsoft Visual Studio. The version of Visual Studio should match the version used when building SICStus Prolog. For this reason a number of variants of SICStus Prolog is released, each built with a separate version of Visual Studio.
Microsoft offers free editions of Visual Studio and its C compilers. It is probably possible to make these work as well but they may require other tools or downloads.
There are several alternative ways to have your installer install the Microsoft C libraries. See the Microsoft documentation for details, or contact SICStus Support.
Building installers for Windows (or for any platform) is complex. There are several tools available, some of them free, that can help with the task. The SICStus Prolog installer is built using the free WiX toolset from Microsoft.