ELKS and MINIX also are not completely open source
OS's in that some
features like
self compilation and C Compiler sources are not available.
ELKS was completely open source last time I checked although the license
might not be pure GPL. (GPL isn't the only open source license).
The Minix I have on my 286 michine certainly has a compiler. Since the
same Minix works on an 8086, the compiler is there. The problem is memory
on the 8086 machine isn't quite enough, you need to run the compiler passes
separately.
Eric