[Simh] simh OS X missing dependencies with brew packages

[Mark Pizzolato]

Now that brew is supported for simh package dependencies, it turns 
out that not all simulator functionality can be realized with the current
set of available packages on brew.  Specifically, the besm6 simulator's
front panel needs sdl ttf support.  On Linux (Ubuntu) this is provided 
by the libsdl2-ttf-dev package.  Maybe someone here can influence 
the brew package maintainers to also add the OS X equivalent package 
to brew's library...

- Mark