Specify C standard in compilation

[zheguang]

This patch adds -std=gnu99 flag to gcc to avoid compilation errors due to difference in C standards.

For example, at default gnu++17 on gcc version 15, there is the following compilation error:

driver.c:335:33: error: too many arguments to function ‘tdefs[tnum].loader’; expected 0, have 2
  335 |                                 tdefs[tnum].loader(&o, upd_num);