This is a fork of https://github.com/adafruit/Adafruit-GFX-Library/tree/master/fontconvert I've added the option to specify a set of glyphs to include in addition to the existing options of ending character number or character range. It's invoked by

fontconvert fontfile size -s stringofglyphstoinclude

This generates the bitmaps for only the glyphs in that string (duplicates are ignored) and outputs the minimal range of glyphs that includes all the glyphs in the string.