char is considered signed in gcc (and, inded, in most x86 compilers), which means that its range is -128 to 127.
Therefore, the test str[i] >= 128 will never succeed, because str[i] will never have a value > 127.
You can fix this problem by placing
#include <limits.h>
with other #includes at the beginning of the file, and surrounding the entire "if" statement with
#if !defined(CHAR_MAX) || CHAR_MAX > 127
#endif
That way, it will compile in the test on machines/compilers that treat chars as unsigned chars (range 0 to 255).
The other thing that you can do is declare
void fixline(signed char* str)
at the beginning of fixnum, and also where it is declared in the .h file.