add ugly sdl_helper to automagically find and enable SDL_mixer

[micropolis] / src / tcl / compat / README

This directory contains various header and code files that are
used make Tcl compatible with various releases of UNIX and UNIX-like
systems.  Typically, files from this directory are used to compile
Tcl when the corresponding files aren't present in the system's
library area.  When the whole world is POSIX-ified, this information
should become unnecessary. (?)