Internal vector fonts have been decoded from original Borland CHR fonts and turned to arrays of coordinates; that is, glyphs.
I’m not aware of the legal status of the CHR fonts that accompanied Borland Turbo C and other compilers. Anyhow, even assuming that they are still copyrighted under U.S. law, their glyphs are not; please see this Wikipedia article for details. It is therefore legal to incorporate the glyphs of CHR fonts in SDL_bgi.
Fonts were taken from Borland Turbo C++ 3, available here, and converted using the ancillary program tmp/chr_decoder.c.