Compiling this

int inv(int n,char mm[n][n]);
int inv(int n,char mm[n][n]){}

using gcc-2.96-69 and the command

gcc -c zog.c

gives this output

zog.c:2: variable-size type declared outside of any function
zog.c:2: variable-size type declared outside of any function

If you remove the first line (the prototype line) then it will compile
OK (It is exactly the same as one of the examples given in "info gcc"
--> C Extensions --> Variable Length). 

Also it compiles OK with earlier gcc's such as kgcc.