The compiler used is DJGPP, a port of GCC that comes with a DOS extender, a POSIX-compatible C library and pretty much everything else needed to pretend that you're using a Unix system. Basically Cygwin for DOS.
But regardless of who was first (they'll be pretty close, '89 or so) it was a huge step forward for programming on x86. The fact that it was free made it even more amazing.
