Original sentence: GNU bison is not as portable as YACC as you need other GNU tools like GCC to compile it, though not to use it.Its generated code is not always compatible with ANSI C
Edited sentence: An exception allowing its output to not have to be under the GPL was added by GNU in bison version 1.24.Another problem is that earlier versions of bison required that the outputted code generated had to be under the GPL.This limitation also keeps it from being as cross platform as it could as the generated code is not always compatible with ANSI C compliant tools that do not support the GNU extensions to C.Unlike yacc, GNU bison was written to lock its design into the GNU tools like gcc and as such needs them to compile itself