You can generate an error for variable length arrays using a compiler flag:

-Werror=vla

Variable-length arrays in C89?

GCC always supported variable length arrays AFAIK. Setting -std to C89 doesn't turn off GCC extensions ...

See also:

https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html

http://tigcc.ticalc.org/doc/gnuexts.html#SEC76

Finally,

http://tigcc.ticalc.org/doc/gnuexts.html

'-pedantic' and other options cause warnings for many GNU C extensions. You can prevent such warnings within one expression by writing __extension__ before the expression. __extension__ has no effect aside from this.